Nonostructured polypyrrole impedimetric sensors for ... · Michaelis-Menten constant ... found to exhibit good electronic and spectroscopic properties. ... carriers characteristics

NANOSTRUCTURED POLYPYRROLE IMPEDIMETRIC

SENSORS FOR ANTHROPOGENIC ORGANIC

POLLUTANTS

By

RICHARD ODUNAYO AKINYEYE BSc (Hons), MSc (Ibadan) and MBA (Akure)

A Thesis Submitted in fulfilment of the requirement for the Degree of

Doctor of Philosophy in Chemistry

of

The University of the Western Cape

Supervisor

Dr Priscilla Baker

Co-Supervisor

Prof. Emmanuel Iwuoha

May 2007

Nanostructured Polypyrrole Impedimetric Sensors for Anthropogenic Organic

Pollutants

Richard Akinyeye

KEYWORDS

Nanostructured conducting polypyrrole

Naphthalene sulfonic acid

1, 2- naphthaquinone-4-sulfonic acid

Tungsten oxide- polymer composites

Zirconium oxide- polymer composites

Anthropogenic organic compounds

Spectroelectrochemistry of polypyrrole

Impedance spectroscopy

Chemical sensors.

ii

ABSTRACT

Nanostructured Polypyrrole Impedimetric Sensors for Anthropogenic Organic

Pollutants

Richard Akinyeye

Polypyrrole composites of polyaromatic hydrocarbon sulphonic acids (β–naphthalene

sulphonic acid (NSA) and 1, 2-napthaquinone-4-sulphonic acid (NQS)), as well as those

of transition metal oxides (tungsten (VI) oxide (WO3) and zirconium (IV) oxide

(ZrO2)), were prepared and characterised for use as electrocatalytic sensors. The

polymerization of pyrrole in β–naphthalene sulphonic acid (NSA) gave rise to

nanotubules, nanomicelles or nanosheets polypyrrole (PPy) morphologies depending on

the amount of NSA in the polymer and the polymerisation temperature. Scanning

electron microscopy (SEM) measurements showed that the diameters of the

nanostructured polypyrrole-β-naphthalene sulphonic acid (PPyNSA) composites were

150-3000 nm for the tubules, 100-150 nm for the micelles and 20 nm for the sheets. A

red shift in the UV-Vis absorption spectra of PPy was observed for PPyNSA which is

indicative of the involvement of bulky β-naphthalene sulphonate ion in the

polymerization process. The UV-Vis also showed the existence of polaron and bi-

polaron in the polymer which may be responsible for the improved solubility of

PPyNSA compared to PPy. All the characteristic IR bands of polypyrrole were

observed in the FTIR spectra of PPyNSA, with slight variation in the absolute values.

However, the absence of N–H stretching at 3400 cm-1 and 1450 cm-1 usually associated

with neutral polypyrrole confirms that the polymer is not in the aromatic state but in the

excited polaron and bipolaron defect state. Electrochemical analysis of PPyNSA reveals

two redox couples: a/a′ - partly oxidized polypyrrole-naphthalene sulphonate radical

cation/neutral polypyrrole naphthalene sulphonate; b/b′ - fully oxidized naphthalene

sulphonate radical cation/partly reduced polypyrrole-naphthalene sulphonate radical

anion. The corresponding formal potentials measured at 5 mV/s, Eº'(5 mV/s), are 181 mV

and 291 mV, respectively. Analysis of the amperometric response of GCE/PPyNSA

film to phenol gave sensitivities of 3.1 mA/mole dm-3 with a linear correlation

coefficient of 0.982 for phenol concentrations of 19.8 µM to 139.5 µM. The apparent

Michaelis-Menten constant (Km′) was estimated as 160 µM.

iii

Novel polypyrrole thin film microelectrodes prepared from an aqueous solution of the

sodium salt of 1, 2-napthaquinone-4-sulphonic acid and pyrrole in hydrochloric acid as

the supporting electrolyte was characterized electrochemically for the first time and

found to exhibit good electronic and spectroscopic properties. The modified PPyNQS

consisted of nano micelles with diameters of 50–100 nm. It also exhibited more

pronounced voltammetric redox responses, improved solubility and stronger UV-Vis

absorptions at wavelengths for polarons (380 nm), bipolarons (750 nm) and overlapped

bi-polarons (820 nm) compared to conventional PPy. Voltammetric investigations

showed that the polymer exhibited quasi-reversible kinetics in a potential window of -

400 mV to +700 mV, with a formal potential of 322 mV vs. Ag/AgCl. The diffusion

coefficient was calculated to be 1.02 x 10-6 cm2/s for a thin film with a surface

concentration of 1.83 x 10-7 mol/cm2 and a standard rate constant of 2.20 x 10-3 cm/s at

5 mV/s. Substractively normalised in situ Fourier transform infrared spectroscopy

(SNIFTIR) confirmed the incorporation of the surfactant into the polypyrrole film, and

for the first time structural changes within the polymer were observed and used to

explain the electrochemistry of the polymer. Electrochemical impedance spectroscopy

(EIS) results validated the quasi-reversible kinetics observed in the voltammetric

experiment. The changes in electrical properties of the polymer during electrochemical

p-doping and n-doping were quantified by equivalent electrical circuit fitting.

Impedimetric nanosensor systems for the determination of two anthropogenic organic

pollutants, namely benzidine and naphthalene, were constructed with smart Pt/PPyNQS

nanomaterials.

Analysis of sensor systems containing tungsten oxide or zirconium oxide-modified

polypyrrole showed that nanohybrids of the polypyrrole were generated by the in-situ

polymerisation of pyrrole in acidic solutions. Results from morphological and

spectroscopic investigation confirmed the pattern of metal distribution within the

nanohybrid polymers matrix. However, this class of polymers were devoid of charge

carriers characteristics required for electrocatalytic sensor applications. The thesis

provided justification for the preparation of nanostructured conducting polypyrrole for

use as anodes for the determination of phenol, benzidine and naphthalene.

May 2007.

iv

Declaration

I declare that Nanostructured Polypyrrole Impedimetric Sensors for Anthropogenic

Organic Pollutants is my own work, that it has not been submitted before for any

degree or examination in any other university, and that all the sources I have used or

quoted have been indicated and acknowledged as complete references.

Richard Odunayo Akinyeye May 2007

Signed: ……………………………..

v

ACKNOWLEDGEMENTS

Every worthwhile destination requires a journey. Similarly, every journey must have a

destination. My arrival at this destination has been made possible only through the

assistance, guidance and prayers of so many people who are hereby gratefully

acknowledged. Firstly, I will like to give the Almighty God all the glory, honour and

adoration for seeing me through this great journey. As we often say in the laboratory,

that the “road to Nigeria from South Africa is far”. With His presence, it has really

been an interesting and fulfilling venture.

To my supervisors, Dr. Priscilla Baker and Professor Emmanuel Iwuoha, I say a big

thank you for your untiring efforts and interest in this work. You introduced me to the

world of Electrochemistry and ensured that I receive water, care and nourishment

from time to time. Your confidence in me was indeed a great inspiration. From time

to time, you are ever willing to attend to my “don’t knows”. The fruit of your support

is evident, and I am indeed very grateful.

To the Department of Chemistry, University of the Western Cape, Bellville, South

Africa, the Head of the department, Prof. Farouk Ameer and all the members of staff,

I say a big thank you for the good and cordial working relationship I enjoyed during

my studies.

To the National Research Foundation (NRF) of South Africa, thank you for awarding

me a PhD bursary. To the University of the Western Cape, thank you for providing

me with postgraduate conference grant to present my work at ICMAT 2007 in

Singapore.

This study would not have been completely satisfying without the cordial and good

working relationship I had with my colleagues in the Sensor Research Laboratory,

including Vernon, Michael, Immaculate, Sipho, Omotayo, Joseph, Everlyne,

Nicolette, Munkombwe, Fanelwa, Nurali, Lawrence, Jasmina, Leslie, and others.

Your contributions are highly appreciated. The leadership role and cooperation from

our post-doctoral fellows; Mantoa, Amir, Raju, Anna and Tesfaye; are gratefully

acknowledged.

vi

I wish to thank all the ministers and members of the Church of God where I derived

spiritual nourishment. The Redeem Christian Church of God, Victory Center, and

lately, Household of God Centre: you served as a river to keep me evergreen. Most

especially, Pastor and Pastor (Mrs) Sola Oduwole, I am indeed very proud of you.

My brothers and friends both here in South Africa and in Nigeria, I say thank you for

always being there for me from time to time. Tokunbo Akinyeye, Samuel Akinyeye,

Joseph Akinyeye, Martins Akinyeye, Mr & Mrs Toye Omiyale, Kole Amigun, Segun

Akinyemi, Segun Adelana, Lanre Fatoba, Lekan Babajide, Kayode Odunayo, Segun

Ogundele, etc. (you are too many to be all mentioned by name), your contributions

are not unnoticed. Thank you.

I am grateful to the management of the University of Ado-Ekiti, Nigeria for granting

me the permission to undertake this study.

Finally, to all my family members, I say thank you. I am particularly grateful to my

parents, “Mrs Beatrice Akinyeye and Mr Gabriel Rotibi Akinyeye” for my education

and upbringing. It is delightful to see you alive as I progress in life. Painfully, Daddy

passed away at the climax of this work, may his soul rest in peace. My dear wife “Mrs

Caroline Modupe Akinyeye” and my children “Damilola Akinyeye, Folakemi

Akinyeye, Emmanuel Akinyeye and Isaac Akinyeye”, I missed you greatly during the

period I was away studying for this degree. I know you missed me more. All my other

family members are equally gratefully acknowledged. Thanks for the love and care.

vii

DEDICATION

This project is dedicated to

The

Almighty God,

My darling wife

Mrs Modupe Caroline Akinyeye

and

My loving children

Damilola, Folakemi, Emmanuel and Isaac

For your prayers, love, understanding and endurance during my absence

from home during this period.

viii

LIST OF SOME PUBLICATIONS

1. Akinyeye, R.0, Sekota, M., Baker, P. & Iwuoha, E. (2006). Chemical

Synthesis And Morphology of β-Naphthalene Sulphonic Acid Doped

Polypyrrole Micro/Nanotubes, Fullerenes, Nanotubes & Carbon

Nanostructures, 14, 49-55.

2. Akinyeye, R.O., Michira, M., Sekota, M., Ahmed, A-Al, Baker, P. &.

Iwuoha, E. (2006): Electrochemical Interrogation and Sensor Applications of

Nanostructured Polypyrroles, Electroanalysis, 18(24), 2441 – 2450.

3. Akinyeye, R.O., Michira, M., Sekota, M., Ahmed, A-Al, Tito, D. Baker,

P.G.L., Brett, C.M.A., Kalaji, M. & Iwuoha, E. (2007). Electrochemical

Synthesis and Characterization of 1, 2-Napthaquinone-4-Sulphonic Acid

Doped Polypyrrole, Electroanalysis, 19(2-3), 303–309.

4. Michira, M., Klink, M., Akinyeye, R.O., Somerset, V., Sekota, M., Ahmed,

A-Al, Baker, P.G.L. & Iwuoha, E.I. (2007). Anthracene Sulphonic Acid-

Doped Polyanilines: Electrodynamics and Application as Amperometric

Peroxide Biosensor, Chapter 5 in Recent Advances in Analytical

Electrochemistry, 0 – 00, ISBN: 978-81-7895-274-1, edited by Kenneth I.

Ozoemena (in press).

5. Akinyeye, R.O., Michira, M., Botha, S., Baker, P. & Iwuoha, E. (2007).

Electrocatalytic Sensor Applications of Nanostructured Polypyrroles and

Polythiophenes, Chapter 4 in Recent Advances in Analytical Electrochemistry,

0 – 00, ISBN: 978-81-7895-274-1, edited by Kenneth I. Ozoemena (in press).

6. Akinyeye, R.O., Klink, M., Ahmed, A-Al, Ignaszak, A., Baker, P., & Iwuoha,

E. (2007). Impedimetric Applications of Nanostructured Conducting 1, 2-

Naphthaquinone-4-Sulphonated Polypyrroles for the Determination of

Benzidine and Naphthalene, ICMAT 2007 Conference Paper in

“Encyclopedia of Advanced Materials: Science and Engineering” (in review).

ix

TABLE OF CONTENTS

Title page i

Keywords ii

Abstract iii

Declaration v

Acknowledgements vi

Dedication viii

List of Publications ix

Table of contents x

List of Figures xvii

List of Tables xxv

Chapter One Introduction

1.1 Intrinsically conducting polymers and concept of doping 1

1.2 Some applications of intrinsically conducting polymers 8

1.2.1 Polypyrrole and its applications in sensors 10

1.3 Research Objectives 11

1.3.1 Preamble 11

1.3.2 Background information and Motivation 12

1.3.3 Objectives 14

1.3.4 Methodology 14

1.3.4.1 Preparation of ICP nanomaterials and sensors 14

1.3.4.2 Characterization and application of ICP nanomaterials

and sensors 15

1.4 Thesis layout 16

x

References 17

Chapter Two Literature review

2.0 Introduction to Nanostructured Conducting Polymers 26

2.1 Preparation methods for conducting polymers of PANi, PPy and PTh 28

2.2 Strategies for production of ‘nanostructurised conducting

polymers/Polymer composites’ 31

2.3 Polyaniline (PANi) blends/composites 34

2.4 Polypyrrole blends/composites 37

2.5 Polythiophene blends and composites 45

2.6 General properties and factors affecting processability of

nanostructured polyaniline, polypyrroles and polythiophenes. 46

2.6.1 Temperature and ageing process 47

2.6.2 Nature of solvent 47

2.6.3 pH and redox potential of the environment 48

2.6.4 Nature of dopants 48

2.6.5 Other factors 49

2.7 Characterization of nanostructured PANi, PPy’s and PTh’s 49

2.7.1 Morphology characterization 49

2.7.2 Electrochemical characterization 51

2.7.3 Spectroscopic characterization 52

2.8 Application of NCPs in sensors 53

2.9 Future challenges in the scope of conducting polymer applications 59

2.10 Chemical sensors for anthropogenic organic pollutants. 61

2.10.1 Surfactant modified polypyrrole chemical sensors 64

2.10.2 Transition metal oxide modified polypyrrole sensors 66

xi

2.10.3 Polycyclic aromatic hydrocarbons (PAHs) 67

References 72

Chapter Three Analytical Techniques and Experimental Procedures

3.1 Analytical techniques 91

3.1.1 Electrochemical techniques 91

3.1.1.1 Cyclic Voltammetry 91

3.1.1.2 Oyster-young square wave voltammetry (OSWV) 97

3.1.1.3 Differential pulse voltammetry 98

3.1.1.4 Electrochemical Impedance Spectroscopy 100

3.1.1.4.1 Electrical circuit elements 106

3.1.1.4.2 Impedance modeling using equivalent

electrical circuit 109

3.1.2 Spectroscopic techniques (UV-Vis, FTIR, SNIFTIRS) 113

3.1.2.1 Ultra Violet-Visible spectroscopy (UV-Vis) 113

3.1.2.2 Fourier Transform Infra Red (FTIR) 114

3.1.2.3 Subtractively Normalized Interfacial Fourier

Transform Infrared Spectroscopy (SNIFTIRS) 114

3.1.3 Morphological technique (SEM) 115

3.2 Chemical synthesis procedures and characterization of polypyrrole

nanomaterials. 115

3.2.1 Chemicals 116

3.2.2 Chemical synthesis of β-naphthalene sulphonic acid doped

polypyrrole (PPyNSA); polypyrrole from distilled water

(PPyDW); and polypyrrole from HCl (PPyHCl) 116

xii

3.2.3 Chemical synthesis of 1, 2-naphthaquinone-4-sulfonate doped

polypyrrole (PPyNQS) 119

3.2.4. Chemical synthesis of metal oxide doped polypyrrole (PPyMO)

using tungsten oxide (WO3) and Zirconium oxide (ZrO2) 120

3.3 Electrochemical synthesis and characterization procedures

3.3.1 Apparatus 121

3.3.2 Electrochemical synthesis procedures of modified polypyrrole 121

3.3.3 Electrolyte and potential window for polypyrrole synthesis 121

3.3.4 Polypyrrole electrosynthesis from aqueous solution of HCl

and product characterization. 123

3.3.5 Polypyrrole electrosynthesis from aqueous solution of

β-naphthalene sulphonic acid and product characterization. 124

3.3.6 Polypyrrole electrosynthesis from aqueous solution of 1, 2-

naphthaquinone-4-sulfonate and product characterization

(PPyNQS) 124

3.3.7 Polypyrrole electrosynthesis from aqueous solution of metal

oxide of tungsten oxide and zirconium oxide 125

3.3.8 Electrochemical characterization of chemically synthesised

polypyrrole 125

3.4 Sensor development 126

3.4.1 Chemicals 126

3.4.2 Phenol sensing with GCE/PPYNSA (Amperometry) 126

3.4.3 Benzidine sensing with Pt/PPyNQS (Impedimetry) 126

3.4.4 Naphthalene sensing with Pt/PPyNQS (Impedimetry)

References 128

xiii

Chapter Four Results and discussion 1

Morphology, Spectroscopy, Electrochemistry and Application of nanostructured

polypyrrole-β-naphthalene sulphonic acid (PPyNSA) composites

4.0 Introduction 132

4.1 Polymerization yield of PPyNSA 134

4.2 Morphology of PPyNSA 138

4.3 UV-Vis absorption and solubility of PPyNSA 142

4.4 FTIR spectral studies 144

4.5 Electrochemical studies on PPyHCl and PPyNSA 147

4.5.1 Chemically synthesised PPyNSA 147

4.5.1.1 Voltammetric studies of GCE/PPyNSA systems 147

4.5.1.2 Electrode kinetics of GCE/PPyNSA systems 152

4.5.1.3 Impedance studies PPyNSA systems 158

4.5.2 Electrosynthesised PPyHCl and PPyNSA 160

4.5.2.1 Voltammetric studies on electrosynthesised PPyHCl

and PPyNSA 160

4.5.2.2 Kinetic studies on electrosynthesised PPyHCl 169

4.6 Amperometric response of GCE/PPyNSA to phenol 170

Conclusions 172

References 173

Chapter five Results and discussion 2

Morphology, Spectroscopy, Electrochemistry and Applications of novel

polypyrroles-1, 2-napthaquinone-4-sulphonate (PPyNQS) composite


xiv

5.1 Chemical synthesis and yield optimization 179

5.1.1 Yield and morphological properties of PPyNQS 181

5.1.2 Spectroscopic properties of PPyNQS 184

5.2 Electrochemical investigation 189

5.2.1 Chemically synthesized PPyNQS 189

5.2.2 Electrochemically synthesized PPyNQS 189

5.2.2.1 Voltammetric investigation 190

5.2.2.2 Kinetic analysis of the Pt/PPyNQS system 198

5.2.2.3 Impedance spectroscopic investigation of Pt/PPyNQS 201

5.3 Modeling of the electrochemical and impedimetric properties of PPyNQS 207

5.4 Test application of Pt/PPyNQS for benzidine and naphthalene 211

Conclusions 218

References 219

Chapter Six Results and discussion 3

Spectroscopic and morphological studies of polypyrrole composites with metal

oxides (PPyWO3 and PPyZrO2)


6.1 Chemical synthesis of (PPyWO3 and PPyZrO2) 225

6.1.1 Yield pattern of metal oxide modified polypyrroles 225

6.1.2 Morphological and EDX examination 226

6.2 Spectroscopic properties of metal oxide modified polypyrroles 229

6.2.1 UV-Vis Spectroscopy 229

6.2.2 FTIR spectral studies 231

Conclusions 236

References 237

xv

Chapter Seven Conclusions and Recommendations

7.1 Conclusions 238

7.2 Main scientific contributions of the dissertation 239

7.3 Recommendations for future work 240

7.4 Output from the dissertation 241

7.4.1 Contributions at conferences 241

7.4.2 Manuscripts and publications from dissertation 244

xvi

LIST OF FIGURES Figure 1.1: Structures of polyacetylene (PAc), polypyrrole (PPy),

polyphenylene, polythiophene (PTh), polyaniline (PANi). 4

Figure 1.2 Conductivity of some metals and doped conjugated polymers 5

Figure 1.3: Band structure in an electronically conducting polymer 7

Figure1.4: Postulated oxidation-reduction processes for (I)

polypyrrole, (II) polythiophene and (III) polyaniline 9

Figure 2.1: SEM and TEM images of Polypyrrole nanowires under different synthesis

conditions 33

Figure 2.2: Schematic diagram of PANi in different oxidation states namely, LM

(insulator) as (A), EM-base (insulator) as (B) and PE (insulating) as (C) 35

Figure 2.3: Generalized scheme for the polymerization of polypyrrole 39

Figure 2.4: Scheme for the electropolymerisation of polypyrrole 40

Figure 2.5: Scheme for the structures of polypyrrole showing the

non-degenerate: aromatic (a) and quinoid (b) configurations;

and degenerate (oxidised forms): a polaron defect (c), and a

bipolaron defect (d) configurations 41

Figure 2.6: The polypyrrole oxidation and reduction scheme 44

Figure 2.7: SEM micrographs of as-synthesized PPy-chloride nanotubes

from ethanol with V2O5 as sacrificial template 51

Figure 2.8: Schematic representation of electrochemical oxidation

and reduction of a polypyrrole film 55

xvii

Figure 2.9: Scheme for the mediation process of an immobilized

polymeric film at an electrode surface for an electroactive analyte 56

Figure 2.10: Scheme for the working principle of an electrochemical sensor 61

Figure 2.11: Scheme for the metal oxide semiconductor sensor 66

Figure 2.12: Scheme of a conductometric based sensor using a conducting

polymer material 67

Figure 2.13: Structural representations of some PAHs included in the USEPA list 69 Figure 3.1: Typical electroanalytical instrument used for cyclic voltammetry 92

Figure 3.2: Typical cyclic voltammogram for the electrochemical oxidation

and reduction process 94

Figure 3.3: Typical OSWV for PPyNSA film containing the forward,

reverse and reverse currents 98

Figure 3.4: Typical potential-current curve for the anodic and cathodic differential

pulse voltammetric scan of polypyrrole in 0.1 M LiClO4 at a scan rate

of 5 mV/s and 25 mV amplitude. 99

Figure 3.5: Sinusoidal current response to potential perturbation as a

function of time 101

Figure 3.6: Typical Nyquist plot with impedance vector and a typical Nyquist plot

of Ferricyanide solution on platinum electrode 105

Figure 3.7: Typical Bode plot of Ferricyanide solution on platinum electrode

showing variation of impedance and phase angle with changes in

frequency 106

xviii

Figure 3.8: Equivalent circuit of a capacitor and a resistor in parallel 106

Figure 3.9: General representation of an electrochemical cell 110

Figure 3.10: Randles circuit for a simple electrochemical cell 112

Figure 4.1: Scheme for the ionic form of β–naphthalene sulphonic acid

doped polypyrrole 134

Figure 4.2: Scheme for the polymerization of polypyrrole via

pyrrole-complex intermediates using the radical cations and

the radical species that are free of naphthalene sulphonate specie 136

Figure 4.3: Typical SEM images of different polypyrrole synthesized with

o/m 0.2 at 25 oC (a) PPyNSA nano/microtubes and fibers from

d/m 1, (b) PPyNSA nano/microtubes from d/m 0.8, and(c)

PPyNSA microfibers from d/m 0.8 139

Figure 4.4: Typical SEM images of polypyrrole nano/micro sheets synthesized

with o/m 0.2 at 25 oC in the absence of NSA dopant (PPyDW) 139

Figure 4.5: Typical SEM images of different polypyrrole synthesized at 0 oC

(a) nano/microtubes and fibers from PPyNSA (d/m 0.8; o/m 0.2),

(b) nanomicelles from PPyNSA (d/m 0.8; o/m 1.0),

(c) nanosheets from PPyDW (o/m 0.2) 141

Figure 4.6: UV-Vis results for PPyNSA prepared under different synthesis

conditions: Fig. 4.6A: PPyNSA (d/m, o/m): (a) [0.5, 0.2] and

(b) [0.8, 0.2], Fig. 4.6B: PPyNSA [d/m 0.8] from o/m ratios:

(a) 0.2, (b) 0.5 and (c) 1.0 143

Figure 4.7: FTIR spectra of polypyrroles in KBr medium for: (a) PPyDW,

(b) PPyNSA (d/m 0.8; o/m 1.0) and (c) PPyNSA (d/m 0.8; o/m 0.2) 145

xix

Figure 4.8: (a) Multi–scan rate voltammograms in 0.1 M HCl for (a) PPyNSA

(d/m 0.8; o/m 0.2) at scan rates of 10, 20, 30 mVs-1; (b) PPyNSA

(d/m 0.8; o/m 1.0) at scan rates of 5, 10, 20 mVs-1 in 0.1M HCl 149

Figure 4.9: Typical anodic SWV of PPyNSA (d/m 0.8; o/m 1.0) at

frequencies of 2, 3, 4 and 5 Hz. showing (a) the forward and

reverse waves (b) the net square wave responses 151

Figure 4.10: Typical anodic DPV of PPyNSA (d/m 0.8; o/m 1.0) at scan

rates of 5 and 10 mV/s 152

Figure 4.11: Plot showing the variation of the modulus of both the anodic

and cathodic peak currents with square root of scan rates for

(a) GCE/PPyNSA (d/m 0.8; o/m 0.2) system and (b) GCE/PPyNSA

(d/m 0.8; o/m 1.0) in 0.1 M HCl 155

Figure 4.12: Plots of the real impedance (Z) and capacitance (C) data of

PPyNSA (d/m 0.8, o/m 1.0) system showing low frequency

behaviours between -600 mV to 700 mV 159

Figure 4.13: Plots of the real impedance (Z) data obtained at 0.1 Hz for

PPyNSA (d/m 0.8, o/m 1.0), PPyNSA (d/m 0.8, o/m 0.2) and

PPyDW at different potentials 160

Figure 4.14: (a) The polymerization voltammograms of Pt/PPyHCl (20 cycles)

from 0.1 M Pyrrole in 0.1 M HCl at 50 mV/s and (b) multi-scan

rate voltammograms for electropolymerized Pt/PPyNSA at

10 to 50 mV/s 162

Figure 4.15: (a) The polymerization voltammograms of GCE/PPyHCl (30 cycles)

from 0.1 M Pyrrole in 0.1 M HCl at 50 mV/s and (b) multi-scan rate

voltammograms for electropolymerized GCE/PPyNSA at

10 to 100 mV/s 164

Figure 4.16: Polymerization voltammograms of GCE/PPyNSA (15 cycles) from

xx

0.1 M Pyrrole and 0.05 M NSA in 0.1 M HCl at 50 mVs-1 and

(b) multi scan rate voltammogram for the electropolymerized

GCE/PPyNSA at 5 to 400 mV/s

166

Figure 4.17: (a) Polymerization voltammograms of GCE/PPyNSA (10 cycles)

from 0.1 M Pyrrole and 0.05 M NSA in 0.1 M HCl at 20 mVs-1

showing overoxidation current pattern, (b) multiscan

voltammograms of overoxidised GCE/PPyNSA in 0.1 M HCl 168

Figure 4.18: Plot showing the variation of the modulus of both the anodic

and cathodic peak currents with square root of scan rates for

Pt/PPyHCl system in 0.1 M HCl 170

Figure 4.19: Graph of the square wave voltammetric response of GCE/PPyNSA

to different concentrations of phenol in 0.05 M HCl 171

Figure 4.20: Calibration plots of GCE/PPyNSA sensor for phenol concentrations 172

Figure 5.1: Ionic form of polypyrrole 1, 2-naphthaquinone-4-sulphonate

(PPyNQS) smart nanomaterials 180

Figure 5.2: SEM micrographs of dry powder PPyNQS showing the typical

fibrous-micellic structures obtained from (a) PPyNQS

[d/m 0.05; o/m 0.2] and (b) PPyNQS [d/m 0.05; o/m 1.0] 183

Figure 5.3: SEM and EDX analysis for dry powder of PPyNQS (top)

compared with that from naphthalene sulphonic acid doped

polypyrrole (PPyNSA) (below) 184

Figure 5.4: FTIR spectra of polypyrroles in KBr medium for: (a) PPyDW,

(b) PPyNQS(d/m 0.05; o/m 0.2) and (c) PPyNQS (d/m 0.05; o/m 1.0) 185

Figure 5.5: Full SNIFTIRS spectra of PPyNQS at 100 mV potential intervals

from 0 to 600 mV, vs calomel electrode 187

xxi

Figure 5.6: Normalised SNIFTIRS spectra of PPyNQS showing the enlargement

of the finger print region

188

Figure 5.7: UV-Vis results for PPyNQS compared with that of PPyDW 189

Figure 5.8: Typical voltammogramms for (a) the polymerisation of Pt/PPYNQS

from 0.1 M Py + 0.01 M NQS in 0.05 M HCl. (25 cycles) at 50 mV/s

and (b) multi–scan rate voltammograms of the thin film in 0.05 M HCl

at scan rates of 5, 10, 15, 20, 25, 30, 40 and 50 mV/s 191

Figure 5.9: (a) Plot of variation of anodic and cathodic peak currents with square

root of the scan rates and (b) plot showing the variation of the modulus

of both the anodic and cathodic peak currents with square root of

scan rates of Pt/PPyNQS in 0.05 M HCl 194

Figure 5.10: (a) Plot showing the variation of cathodic peak potentials with scan

rates and (b) plot of peak separations versus scan rates of

Pt/PPyNQS in 0.05 M HCl at a 1.6 mm diameter Pt electrode

at a temperature of 25 °C based on the data from Fig. 5.8b 196

Figure 5.11: Differential pulse voltammograms for the anodic and cathodic

wave difference for Pt/PPyNQS film in 0.05 M HCl using a

scan rate of 5 mV/s and 50 mV pulse amplitude 197

Figure 5.12: Square wave voltammogram for the anodic and cathodic wave

difference for Pt/PPyNQS film in 0.05 M HCl using frequency of

15 Hz and 25 mV amplitude 198

Figure 5.13: Plot of the peak current dependence on scan rate for Pt/PPyNQS

prepared from 0.1 M Py + 0.01 M NQS in 0.05 M HCl and

characterized in 0.05 M HCl 200

Figure 5.14: Complex plane impedance plots of PPyNQS thin film electrode

xxii

at 50 mV vs. Ag/AgCl in 0.05 M HCl during (•)step-by-step

oxidation and (o) subsequent reduction 202

Figure 5.15: Plot of interfacial capacitance versus potential for the

oxidation and reduction of the PPyNQS thin film 205

Figure 5.16. Model illustrating the alignment of charges at different oxidation

states of polypyrrole: (a) neutral polymer and (b) oxidized polymer 206

Figure 5.17: Equivalent electrical circuit describing the electrical components

of Pt/PPyNQS in an electrolyte medium of 0.05 M HCl 208

Figure 5.18: Typical Nyquist plot (top), Bode plot (bottom) for

Pt/PPyNQS system 209

Figure 5.19: Plot of the interfacial impedance and capacitance at different

perturbation potentials of the Pt/PPyNQS electrode 210

Figure 5.20: Plot of changes in real impedance with increasing concentrations

of benzidine at the bulk polymers region () and at the interface () 211

Figure 5.21: Plot of changes in capacitance with increasing concentrations of

benzidine at the interface with insert showing the calibration curve

for the linear region 214


benzidine at the bulk polymers region in () and the straight line

showing the regression line 215

Figure 5.23: Plot of changes in impedance with increasing concentrations of

benzidine at the bulk polymers region in () 216


naphthalene at the interface with insert showing calibration curve

xxiii

for the linear region 217

Figure 6.1: SEM micrographs of dry powder of metal oxide modified

polypyrrole showing the typical fibrous nanostructures from

(a) PPyWO3 [d/m 0.8; o/m 0.2] and (b) PPyZrO2 [d/m 0.8; o/m 0.2] 228

Figure 6.2: UV-Vis results for metal oxide modified polypyrrole prepared

under different synthesis conditions: Fig. 6.2A: PPyWO3 from

o/m 0.2 and 1.0; and Fig. 6.2B: PPyZrO2 from o/m 0.2 and 1.0 231

Figure 6.3: FTIR spectra of metal oxide modified polypyrroles in KBr medium,

Fig. 6.3A: (a) PPyDW, (b) PPyWO3 (d/m 0.8; o/m 0.2) and

(c) PPyWO3 (d/m 0.8; o/m 1.0); and Fig. 6.3B: (a) PPyDW,

(b) PPyZrO2 (d/m 0.8; o/m 0.2) and (c) PPyZrO2 (d/m 0.8; o/m 1.0) 234

xxiv

LIST OF TABLES

Table 1.1: Names and repeat units of the most widely studied

conducting polymers 3

Table 2.1: Chemical structure of the 15 European Union (EU) priority PAHs 70

Table 3.1: Table of relative amounts of materials used for chemical

synthesis of PPyNSA, PPyDW and PPyHCl 118

Table 3.2: Table of relative amounts of materials used for chemical synthesis

of PPyNQS 120

Table 3.3: Table of relative amounts of materials used for chemical

synthesis of PPyWO3 121

Table 3.4: Table of relative amounts of materials used for chemical synthesis

of PPyZrO2 121

Table 4.1: Yield/Temperature profile for doped and undoped PPy 134

Table 4.2: Yield of various polypyrroles prepared under different synthesis

conditions 137

Table 4.3: Major shifts of bands (cm-1) in FTIR spectra of PPyDW,

PPyNSA (d/m 0.8, o/m 0.2) and PPyNSA (d/m 0.8, o/m 1.0)

from undoped polypyrrole major bands (Geetha & Trivedi) 146

Table 4.4: Summary of estimates of kinetic parameters for PPyNSA

[d/m 0.8, o/m 1.0] on GCE based on n = 1, and scan rate measurements

from 10 – 100 mV/s 157

xxv

Table 5.1: Yield profile for chemically synthesised PPyNQS at different

dopant and oxidant concentration ratios 181

Table 5.2: Major shifts of bands (cm-1) in FTIR spectra of PPyDW,

PPyNQS (d/m 0.05, o/m 0.2) and PPyNQS (d/m 0.05, o/m 1.0)

from undoped polypyrrole major bands (Geetha & Trivedi) 186

Table 5.3: Analysis of the oxidative impedance data at the conductive

polymer electrode 203

Table 5.4: Analysis of reductive impedance data at the conductive

polymer electrode 203

Table 6.1: Yield of metal-oxide modified polypyrroles prepared under

different synthesis conditions 226

Table 6.2: Comparative trend of elemental composition (C, S, O, W, Zr, others)

in different modified polypyrroles prepared at from d/m ratio of 0.8

and o/m ratio 0.2 by EDX spectroscopic analysis 229

Table 6.3: Major shifts of bands (cm-1) in FTIR spectra of PPyDW, PPyWO3

(d/m 0.8, o/m 0.2) and PPyWO3 (d/m 0.8, o/m 1.0) from undoped

polypyrrole major bands (Geetha & Trivedi) 235

Table 6.4: Major shifts of bands (cm-1) in FTIR spectra of PPyDW, PPyZrO2

(d/m 0.8, o/m 0.2) and PPyZrO2 (d/m 0.8, o/m 1.0) from

undoped polypyrrole major bands (Geetha & Trivedi) 236

xxvi

Chapter One

Introduction

This chapter gives a brief introduction to intrinsically conducting polymers, the

concept of doping and conductivity especially in polypyrroles. The project proposal,

objectives and methodology that was adopted is presented.

1.1 Intrinsically conducting polymers and concept of doping

The current day science aims at using nanostructured-conducting polymers to boost

the development of exciting opportunities in modern day science and technology.

Towards achieving this objective, novel materials and technologies, new ideas,

applications and techniques will always be a challenging route to explore new

possibilities. The current interest in the world of conducting polymer science evolved

from the discovery that the erstwhile inactive, non-conductor polymers mostly used as

electrical insulators could still be obtained in a conducting state through the presence

of certain additives in the polymer that enhances the conductivity [1-15]. This

commonly is referred to as “doping the polymer” and it provides increase in

conductivity of several orders of magnitude from the semiconductor regime. This

doping terminology in conductive polymers is slightly different from its conventional

use in semi-conductor physics, since considerably higher concentration (of up to 33%)

are employed in the former [7].

This new class of polymer known as intrinsically conductive polymers (ICPs) or

electronic (electroactive) polymers (EP) combines the mechanical and chemical

properties of insulator polymers with the electrical and optical properties of inorganic

semiconductors and metals [2]. This class of material is completely different from

“conducting polymers” which are merely a bulk material generated from the physical

mixture of a nonconductive polymer with a conducting material such as a metal or

carbon powder that has been uniformly dispersed. Intrinsically conducting polymers

offer a unique combination of ion exchange characteristics and optical properties that

1

make them distinctive. They are readily oxidized and reduced at relatively low

potentials, and the redox process is reversible and accompanied by large changes in

the composition, conductivity and colour of the material. In 1969, the use of

electronic polymers was proposed as light emitting material, however, its first use as

light emitting diode was launched in 1991 [3]. This involved the combination of

carbon and metal filled polymers as mouldable semiconductors, as photoreceptors in

electro photographic copying machines and some other applications.

Great publicity into the potentials in organic polymers actually came up in 1977,

when the conductive properties of the alternating-bond in the conjugated polymer of

trans-polyacetylene were discovered. Hideki Shirakawa et al. in 1971 had reported the

enhanced conductivity of polyacetylene observed in his laboratory following an

accidental addition of excess Ziegler-Natta catalyst as dopant to the pristine semi

conducting polyacetylene [4]. The development generated a lot of curiosity amongst

scientist like Heeger and MacDiarmid who found that the conductivity of

polyacetylene could be enhanced by exposure to oxidizing agents such as iodine

vapor, AsF5, NOPF6 (p-doping) or reducing agents such as sodium naphtide from 10-9

S cm-1 to 105 S cm-1, an increase of well over ten million times [4]. In particular,

exposure of iodine vapor to polyacetylene generated increased conductivity of about

ten million times (from 10-3 S m-1 to 3000 S m-1) [2]. It was found that the polymer

could be doped either chemically or electrochemically to the metallic state and

thereby transformed into a good electrical conductor almost comparable to that of one

single copper crystal. The recognition of these efforts by the world scientific

community was the Nobel Prize in Chemistry awarded to Professors Heeger A, J.,

MacDiarmid A.G. and Shirakawa H., in 2000 for their research in that field.

Thereafter, many new conducting polymers and their derivatives were discovered and

applied for different electronic applications in different fields. These include organic

polymers such as polyaniline (PANi), polypyrrole (PPy) polythiophene (PTh), poly-

(para-phenylene), poly-(phenylenevinyl-ene), polyfuran and other poly-

(heteroaromatic vinylenes) [5]. Table 1.1 shows the idealized structure of the mostly

studied conducting polymers. The structural feature common to conducting polymers

is their alternating single and double bond lattice structure that allows for the transfer

of charge carriers upon excitation of electron.

2

Table 1.1: Names and repeat units of the most widely studied conducting

polymers

Figure 1.1 shows the chemical structures of some conducting polymers. They are one-

dimensional conductors as the electrons/holes travel mainly through the linear

backbone. Doping is either the addition of electrons (reduction reaction, n-doping) or

the removal of electrons (oxidation reaction, p-doping) from the polymer. The un-

paired л electron per carbon atom in the conjugated polymers is only loosely bound

and because they are covalently bound they have restricted span of movement. These

л bonds in conjugated polymers are highly susceptible to chemical or electrochemical

oxidation or reduction. Positive charges (holes) and negative charges (electrons) move

to opposite directions. Once doping has occurred, the electrons in the pi-bonds are

able to "jump" around the polymer chain, as they are now intrinsically mobile.

During this process, the polymer, which is an insulator or semiconductor, is converted

to metallic polymer, frequently called “synthetic metal” because they present electric,

electronic, magnetic and optical properties of metals [5-12].

3

CC

CC

CC

CC

HH

HH

HH H

HH H

Polyacetylene

N

H

N

H

N

H

N

H

N

H

Polypyrrole

Polyphenylene

S

S S

SS

Polythiophene

N N

HH

Y

N N

1-Y

Polyaniline

Figure 1.1: Structures of polyacetylene (PAc), polypyrrole (PPy), polyphenylene,

polythiophene (PTh), polyaniline (PANi).

It was anticipated in the late 1970s to early 1980s, that synthetic conducting polymers

would soon replace metals in many applications. These projected advances, expected

4

by chemist, for these materials have proven to be somewhat illusive to date [6]. Figure

1.2 shows the trend of conductivity in some doped conducting polymers compared

with some metals and semiconductors as presented by Freund & Deore [14].

Figure 1.2 Conductivity of some metals and doped conjugated polymers [14].

The movement of electrons along the polymers molecule produce electric current.

This electric current has been ascribed to the formation of non-linear defects such as

solitons, polarons or bipolarons formed during doping or polymerisation of a

monomer [7, 10, 16]. However the conductivity of the material is limited, as the

electrons have to "jump" across molecules, so for better conductivity the molecules

must be well ordered and closely packed to limit the distance "jumped" by the

electrons.

Positive charges (holes) and negative charges (electrons) move to opposite electrodes.

This movement of charges is actually responsible for the observed electrical

5

conductivity [8]. This is why small particulate and nanostructured conducting

polymer materials with large surface area are desirable for good conductivity.

Doping could be achieved via chemical and electrochemical routes. Introduction of

mobile carriers into the double bonds of the polymer backbone is the source of the

polymers conductivity, and that is why the enumerated properties above are only

intrinsic to the doped state [11, 12]. According to Wikipedia® 2007, the doping

process in ICPs is similar to that of inorganic semiconductors such as silicon which

could be doped with the addition of electron rich atoms such as phosphorous or

electron poor atom such as boron to create an n-type or p-type semiconductor

respectively [8]. This doping action could produce significant effects on the

conductivity of the material, even in concentrations of one part per million. While the

n-type doping is common in inorganic semiconductors, it is very rare in organic

semiconductors. Most of the optical characteristics inherent to inorganic

semiconductors, such as photoemission, photodetection and photocurrent have also

been observed in ICPs [13]. Chemical procedure for n-doping is rarely employed

because of the oxidizing nature of the atmosphere which is rich in molecular oxygen

that has propensity of de-doping (oxidizing to the neutral state) an n-doped polymer

once it is in contact with the atmosphere. The electrochemical route to n-doping is

equally not very common in research because it is not easy to exclude oxygen

completely from a solvent in a sealed cell even when burbled with argon during

synthesis. Thus n-doped conducting polymer is usually not of much commercial value

and thus hardly used [8].

The relatively low ionization energy and high electron affinity of the conjugated

bonds readily generate changes in the electronic structure of the polymer chain. This

change in electronic structure is accompanied with a change in the conductivity of the

polymer. Thus the relative concentration of the charge carriers, solitons i.e. radicals

with unpaired electrons, polarons i.e. couples formed from neutral or charged solitons,

or bipolarons determines the polymers conductivity. The gap between the polymers

valence band and the conduction band determines the relative ease with which

electrons will jump across the gap. In a doped polymer, charge is removed from the

highest occupied molecular orbital (HOMO) while charge is injected to the lowest

unoccupied molecular orbital (LUMO) and this takes place in the mid-gap states [14].

6

These bands stem from the splitting of interacting molecular orbitals of the constituent

monomer units in a manner reminiscent of the band structure of solid-state

semiconductors (Figure 1.3). The smaller the band gap, the higher the doping level in

the polymer.

C O N D U C T IO N B A N D

V A L E N C E B A N D

E n e rg y

b a n d g a p

… … ..L U M O

… … .H O M O

C O N D U C T IO N B A N D

V A L E N C E B A N D

E n e rg y

b a n d g a p

… … ..L U M O

… … .H O M O

Figure 1.3: Band structure in an electronically conducting polymer

The mechanism of charge propagation in conducting polymer using polypyrrole as

example is discussed further in section 2.2. Other factors that influence the

conductivity are impurities, folds in the chain structure, chain ends, and differences

between amorphous and crystalline regions [12].

Conductivity is not only a result of charge transfer along the chain, but is also due to

electron hopping between chains and between different conjugated segments of the

same polymer chain. In addition to these effects that act at a molecular level, electron

transfer between grain boundaries and variations in morphology also dominates bulk

conductivity values. Thus, the conjugated polymers bulk conductivity may be

described by Equation 1.1 [7].

∑=E

evZnσ iii Equation 1.1

where σ = conductivity (S/cm),

ni = number of charges carried by each type i,

Zi = carrier type,

e = electronic charge (1.60 x 10-19 C),

vi = drift velocity of electron (cm/s), and,

E = electric field (V/cm)

7

Eq. 1.1 takes into account different types of charge carrier, which contribute to the

overall observed electrical conductivity in the bulk polymer.

1.2 Some applications of intrinsically conducting polymers

The ICPs have been a big challenge to classical materials in many applications. There

is however the need to temper the enthusiasm generated by the discovery of ICPs as

potential competitor for classical metals and semiconductors, but rather seen as mere

opportunities for new applications [1]. Polymer films of ICPs such as PPy, PTh and

PANi on electrode surfaces can be switched between the oxidized conducting state

and the reduced insulating state by the ingress and egress (i.e. doping and de-doping)

of counter anions [15]. This change is always accompanied with a proportionate

signal (e.g. current, impedance, capacitance, etc) corresponding to the concentration

of the counter ion in solution and the change in conductivity of the polymer.

The oxidation-reduction processes for PANi, PPy and PTh on an electrode surface are

depicted in Fig. 1.4. This reversible charging and discharging property of ICPs is a

useful property employed in the production of various electronic devices,

optoelectronic and biotechnological applications such as rechargeable batteries,

molecular electronics, solar cells, electronic displays, electrodes, diodes, redox

supercapacitors and superconductors, Electromagnetic material interference (EMI)

shielding materials, ion exchange membranes in fuel cells, field effect transistors,

printed circuit boards, electrochemical ionic sensors, use as ion gate membrane for

drug release systems and biosensors, etc [11]. A lot of monographs and reviews have

been documented for different scope of applications [7, 11, 13, 15, 16]. Some of these

application ranges are already commercialised while some are still being developed.

The colour changes during switching of ICPs (conducting states) enable their use in

the manufacture of multichromic displays and electrochromic windows [11]. The

hole-injection properties of the polymers under an applied potential or current enable

their use as flexible light emitting diodes (LED) and light emitting cell (LEC) [11].

8

The ability of ICPs to change chemical properties by chemical, thermal or appropriate

polarisation equally provides them with unique properties for easy changeable

retention behaviour. This property is employed in their use for solid phase extraction

(SPE) based on the sorption of analytes on the conducting polymer coated solid

support. Chromatographic application using ICPs as a new stationary phase for gas

and liquid systems as well as in electrochemical detectors in liquid chromatography

are part of the diverse application of this novel material [16].

NH n

+

A-

NH n

0

+ A-+e-

-e-

I

II

S

+

n

A- +e-

-e-S n

0

+ A-

III

NH

NH

NH

NH

n

NH

NH

NH

NH

+. +.

n

N N N N

A- A-

- 2e-

+ 4H+- 4H+

+ 2 e-

+2e- -2e-

HA

n

Figure1.4: Postulated oxidation-reduction processes for (I) polypyrrole, (II)

polythiophene and (III) polyaniline [15].

Another successful and important application of ICPs is in the construction of gas

sensing arrays (electronic-noses) based on the use of solid contacts with ion selective

electrodes [16].

9

Recently, use of nanostructured conducting polymers as electrode materials has

received remarkable interest owing to improved electrocatalysis [17]. The production

of the electrode materials must therefore take the advantage of using methods that

generates nanostructured materials with good, cost effective and competitive

electroanalytical applications. Section 2.1 presents the strategies for production of

nanostructures such as template free, template assisted and molecular template

assisted methods. These nanostructures are miniatures such as nanowires [18],

nanofibres [19], nanotubes, nanoballs, and nanodots and in particular nanoparticles

[17, 20, 21]. The advantages offered by nanoparticle-modified electrode when

compared to a microelectrode are high effective surface area, mass transport,

catalysis, and control over local environment [22].

1.2.1 Polypyrrole and its applications in sensors

Amongst the ICPs, PPy has excellent environmental stability, easy synthesis, good

conductivity and other intrinsic properties that are of promising applications in

various fields of science and technology [23–30]. However, its application is grossly

limited because it is insoluble and infusible. Various efforts have been made to

address this gross shortcoming that impairs the PPy’s processability so that the

inherent electronic potentials offered by the polymer could be maximally harnessed

[30 -32]. The use of conducting PPy as sensors is derived from the polymers ability to

manifest different conductivities when exposed to different types and concentration of

analytes. It is also well known that the mechanical, physical and chemical properties

of PPy strongly depend on the nature of the dopant anion [33]. The interaction of

gaseous components with deposited PPy films produces changes in colour, mass,

work function or electrical conductivity. The resistance decreases with the gas of

larger electron affinity [17, 34]. The factors responsible for the observed changes in

resistance are ionic interactions, type of functional groups, presence of lone pairs and

electron bridging species [17].

Over the last three decades, it has been known that chemiresistors with ICP layer on

an electrode do respond to a variety of gases and some organic vapours [16]. The

interaction of electrophilic gases attracts electrons from the polymer phase, thus

causing an increase in conductivity whereas nucleophilic gases increase the resistance

10

of the polymer. The use of a conducting polymer modified electrode can be used for

the detection of not only gaseous analytes but also for solution species. The

interaction or adsorption of organic or inorganic molecular species at the

polymer/solution interface may also affect the electronic charge transfer process in

relation to the concentration of the gaseous or liquid analytes. Detection of ammonia

in aqueous solution was reported for polypyrrole [35], and it is noteworthy that the

PPy application was patented shortly thereafter [36]. A comprehensive review of

different chemical sensors for electronic nose systems was reported recently by James

et al. (2005) involving the use of intrinsic (doped) conducting polymers, extrinsic

(composite) conducting polymers based on different transduction technologies [37].

Today, there are a lot of anthropogenic pollutants in our gaseous and aqueous

environment and there is the challenge of controlling the concentration of these

analytes using improved electrochemical techniques. This challenge calls for new

electrochemical sensors that could be used to provide continuous information about

the environment. The synthesis, characterisation and application of nanostructured

PPy modified with different dopants for the determination of some anthropogenic

organic pollutants found in wastewaters shall be investigated in this study. Details of

the possible interfacial interaction between the various analytes and the transducer

(PPy) will be undertaken based on the understanding that the molecules of analytes

are first adsorbed on the surface of the sensor and finally absorbed into the matrix of

the polymer [38, 39]. The research proposal which was the basis of this study is

summarised and presented underneath.

1.3 Research Objectives

1.3.1 Preamble

Specialty polymers, such as native and derivatised polypyrrole (PPy), polythiophene

(PTh) and polyaniline (PANi) are the most popular intrinsically conducting polymers

(ICP) [40, 41]. The research and industrial interest in these ‘organic electronics’ is

due to a good combination of properties, reasonable stability, low cost, ease of

synthesis, and the possibility of tailoring the structures on the molecular scale. There

are numerous attempts to apply high conductivity, electrochromic, catalytic, sensor,

11

redox and other properties of these polymers to different practical needs [41–53].

Their direct application has been however greatly limited because they suffer

processability limitation because of their intractable nature. Of recent, the application

of ICPs has been widened through formation of composites or blends with common

polymers [41, 43-47, 54–56] and other particulate dispersants like SiO2 and SnO2 to

form nanocomposites with better processability [52].

1.3.2 Background information and Motivation

The discovery of carbon nanotubes (CNTs) in 1991 by Sumio Iijima and the inherent

high conductivity, high chemical stability, extremely high mechanical strength and

modulus, led to a new branch of science called nanotechnology [57, 58]. Due to the

ability of CNTs to promote electron transfer, they are used as electrode materials to

prepare sensors and in the study of direct electrochemistry of biological molecules

[59-63]. The same principle is applicable to the ICPs, otherwise called synthetic

metals.

Present day material science deals with microstructures of characteristic morphology

and dimensions between 1 and 100 nm. Current chip technology is also reported to

produce structures with a characteristic dimension of about 100 nm [57]. In building

up such nanostructures, scientist aims at structuring molecules through ingenious

synthesis procedures to build functional superstructures via supramolecular self-

organization. Reduction in particle size of the polymeric materials to the nanometer

level imparts unique properties that make them suitable for chemical and physical

sensing, and the control of the structure of sensing surfaces on the nanometer scale

can be utilized to enhance sensor performance.

Inorganic, organic as well as biological materials can be integrated to functionalized

systems to build up novel nanostructures for application in catalysis, electro-optics,

high performance ceramics as well as chemo and gene therapy. All these will result in

improvements in the areas of electronics, telecommunication and health care. For

example mixing oxide nanoparticles into poly (2-methoxy-5- (2-ethyl hexyloxy)-1,4-

phenylenevinylene) (MEH-PPV) gave rise to enhancement of the current density and

radiance in polymer light emitting diodes (PLEA), and the composites of conjugated

12

polymers and nanoparticles that exhibited useful composition for application in

photonics [55, 56]. Also, in situ polymerization reactions in medium containing

surfactants such as poly- (styrene sulphonic acid) (PSSA), poly-vinyl alcohol (PVA)

or transition metal systems (e.g. vanadium pentoxide) do generate stable

nanocomposites with properties useful for amperometric and impedimetric sensor

devices [55, 56].

Delvaux et al. [56] reported the template synthesis of PANI micro and nanotubes

based on oxidative polymerization of aniline within the pores of particle track-etched

membranes while Wei et al. [58] suggested the possibility of preparation of PANI

nanotube through a self assembly method. This opens the possibility of extending the

technique of nanotube materials to other polymers. These conducting polymers can be

reversibly changed between insulating and conducting states through chemical or

electrochemical doping reactions [41, 48].

The present study proposes to develop novel nano-structured or nanometer scale

sensor devices for environmental analysis of anthropogenic priority organic pollutants

contained in industrial wastewater systems. Among the compounds of interest are

benzenoids such as benzene, toluene, ethylbenzene and xylene isomers (BTEX) that

are also volatile organic pollutants (VOC’s), chlorinated compounds (e.g.

trichloroethylene, [TCE]), polycyclic aromatic hydrocarbons (PAHs); e.g.

naphthalene, fluroanthene, pyrene) and polychlorinated biphenyls (PCBs) and other

priority organic pollutants (POP’s) such as benzidine, phenol and its derivatives.

These culprit organic compounds are known to have varying degrees of toxic,

mutagenic or carcinogenic activities. Chromatographic (TLC, GC, HPLC),

spectroscopic (UV-Vis, IR, MS) or coupled techniques (GC-MS) are presently

heavily relied upon for environmental analysis. The disadvantages are high cost, not

being user friendly and failure to indicate whether the compounds are accessible for

assimilation by living organisms.

The rationale for this proposed study is the need for relatively low cost, miniaturised

and easy-to-use hand-held nanosensor systems for on site application, which to a great

extent maintains sample integrity. Environmental impedimetric and chemosensor

devices containing ICP nanotube films will be developed and applied for real-time

13

determination and speciation of PAHs and VOCs associated with industrial

wastewater effluents.

1.3.3 Objectives

The main aim of this study is to develop a novel strategy for harnessing the properties

of electroconductive polymers in sensor technology by using polymeric

nanostructured blends in the preparation of high performance sensor devices. The

objectives include:

i. To electrochemically and chemically prepare intrinsically conducting polymers

(ICPs) of pyrrole doped with different polyaromatic hydrocarbon sulphonic acid

(PAHSA) and transition metal oxides of WO3 and ZrO2.

ii. To characterize the intrinsically conducting polymers by instrumental,

electrochemical and impedimetric techniques.

iii. To develop high performance nano-sensor devices using the polymers for

determination of some priority organic pollutants (POPs) present in waste waters.

iv. To model the reactivity of the intrinsically conducting polymers sensor systems.

1.3.4 Methodology

1.3.4.1 Preparation of ICP nanomaterials and sensors

In this study processable electrically conducting PPy nanotubes was prepared by in

situ oxidative polymerization of the monomer in acid solutions using PAHSA of

naphthalene sulphonic acid (NSA) and the sodium salt of 1, 2-naphthaquinone-4-

sulphonic acid (NQS) as stabilizing surfactants. Electrosynthetic or chemically

synthesized nanostructural formats of polymer composites doped with transition metal

oxides (WO3 and ZrO2) were equally explored. The resulting nanotubes will

subsequently be blended with polyurethane acting as a compliant, insulating host for

sensor application. The oxidative polymerization will be performed either electro

synthetically to produce self-assembled polymer layers on platinum, gold or glassy

carbon electrode; or chemically to produce polymer pastes using ammonium

peroxydisulphate as oxidant.

14

1.3.4.2 Characterization and application of ICP nanomaterials and sensors

Electrochemical characterization was performed in aqueous medium with the help of

electrochemical workstations. The electrochemical and spectroscopic techniques used

includes:

i. Cyclic voltammetry (CV).

ii. Osteryoung square wave voltammetry (OSW).

iii. Differential pulse voltammetry (DPV).

iv. UV/Vis spectroelectrochemistry.

v. Impedance spectroscopy.

Morphological and structural characterizations were performed on the prepared

nanomaterials with scanning electron microscopy (SEM).

The application of ICP nanotubes sensors/electrodes was undertaken using glassy

carbon, platinum or gold disc electrodes. The sensors was tested for their suitability as

impedimetric and amperometric electrode materials for some anthropogenic pollutants

in wastewater; namely phenol, naphthalene and benzidine. Electrochemical data

generated for the various electrode materials were explored for the analysis of the

pollutants in wastewaters using their reactivity in aqueous systems by EIS and

voltammetry.

Modeling of the reactivities of ICP modified electrodes and sensors involved the

monochromatic stimulation procedure [64-70] for the measurement of ICPs electrode

impedance. The reactivity of electrochemical sensors involve slow preceding

reactions, slow electrode kinetics and diffusion, all of which contribute to the

impedance of electron flow and hence to lower currents. Impedance data was

generated for studying the electrochemical phenomenon at the electrode’s interface by

perturbations with different potentials over a broad frequency range (usually 100 kHz

to 100 mHz) to determine the electrical characteristics such as resistors, capacitors

and inductors of the system. The study did not only involve the Nyquist plots but also

bode plots so that information on change of impedance with frequency could be

obtained for the circuit elements. The electrochemical modeling of the surface

properties of various ICP-modified electrodes involved search for an appropriate

15

equivalent circuit model in order to isolate the different contribution to the overall

response during an electrochemical perturbation [64-70]. Impedance circuits tested

includes:

(a) Series and parallel combination of single equivalent elements.

(b) Series and parallel combination of multiple equivalent elements.

The preparation of various impedance circuits and models for the various nanosensor

systems depends on the appropriate model that fit the physical electrochemistry and

equivalent circuit elements such as:

(c) Models based on purely capacitive system as Nyquist and Bode plots.

(d) Models based on randles cell for the Nyquist plots.

(e) Models based on Warburg impedance.

(f) Models based on mixed kinetic and charge transfer control circuits as

Nyquist and Bode plots.

(g) Models for a failed coating.

Reliable and scalable synthetic methods for nanostructured conducting polypyrrole

with interesting technological and scientific properties for both research and

environmental applications are to be optimised.

1.4 Thesis layout

The thesis is structured into seven chapters. Chapter one introduces the subject of

conducting polymers and how the intrinsic conductivity is generated. The research

proposal, objectives, methodology and techniques used are briefly highlighted.

Chapter two gives a general literature review on the intrinsically conducting polymers

and techniques for the production of nanostructurised conducting polymers. General

properties of polyaniline, polypyrrole and polythiophene are discussed alongside the

factors that influence their stabilities. A review of the characterisation techniques,

electrocatalytic applications and challenges for the future with respect to these

polymers are discussed. Introduction of dopant materials and the anthropogenic

organic pollutants used as analytes is presented.

16

Chapter three gives a brief synopsis on the different analytical techniques and general

experimental procedure for the chemical and electrochemical preparation; and

characterization of the electroactive materials. This gives the necessary methodology

and the applied electrochemical principles for the sensor development.

Chapter four discusses the experimental results obtained from the chemical and

electrochemical preparation and characterization of naphthalene sulphonic acid

doped-polypyrrole (PPyNSA) alongside hydrochloric acid doped polypyrrole

(PPyHCl). The application of the naphthalene sulphonic acid doped-polypyrrole

(PPyNSA) as amperometric sensor for phenol is presented.

Chapter five presents the experimental results obtained from the chemical and

electrochemical preparation; characterization and application of naphthalene 1, 2-

naphthaquinone-4-sulphonic acid doped-polypyrrole (PPyNQS) as impedimetric

sensor for benzidine and naphthalene.

Chapter six presents the experimental results of the preparation and characterization

of metal oxide doped-polypyrrole (PPyWO3 and PPyZrO2) as potential sensor

material.

Chapter seven gives the summary of the main scientific contributions of the

dissertation, conclusions drawn from them and recommendations for future research

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25

Chapter Two

Literature Review

In this chapter, different routes for the preparation of the intrinsically and

nanostructured conducting polymers (NCP) of PANi, PPy and PTh are discussed. The

preparation and blending methods, properties, factors affecting stability and

characterization techniques of the NCPs together with their hybrids/composites are

reviewed. Moreso, a broad review of their applications as electro catalytic sensors is

presented. A review on the doping materials used in preparing modified polypyrrole

chemical sensors is presented vis-à-vis the anthropogenic organic pollutants

investigated in this study.

2. Introduction to Nanostructured Conducting Polymers

An emerging sub discipline of chemical and material science is nanomaterials. It deals

with development of methods for synthesizing nanoscopic particles of a desired

material and the scientific investigations of the nanomaterial obtained. Sumio Iijima

introduced the terms nanotubes and nanowires to the world of science from his

discovery of carbon nanotubes in 1991 [1]. During the past decade, there has been a

great technological focus on the production of nanostructurised materials [1-10]. The

growth of nanostructures is increasing daily because nanomaterials are not only good

conductors of electricity but potential chemically inert matrix for various electron

field emission characteristics [5].

A lot of research findings have been reported on nanoparticles, nanotubes, nanorods,

nanobelts, nanofilms, nanowires, nanocables and nanocomposites, which are all

nanomaterials [1-5, 7, 10]. Infact the number of nanoparticle-based papers published

per year since 1994 to date has been on the increase [10]. It is therefore not surprising

that a lot of resources are being committed to nanotechnology sector by various

organizations and governments in the USA, Japan and many European countries to

fund research that will help industry harness the commercial opportunities offered by

26

this novel technology [10]. According to a report from Lux Research entitled

"Ranking the Nations: Nanotech's Shifting Global Leaders", the U.S., Japan, South

Korea, and Germany dominate today, but Taiwan and China will challenge them for

leadership in the next seven years [11]. The advantages derived from the use of

nanoparticles or NCPs over micro/macroparticles as an electrode material includes

high effective surface area, better mass transport, better catalysis and control over

local microenvironment. A nanoparticle with sizes of about 100 nm has been reported

to produce seventy times magnitude of current when applied in electroanalyses

compared to that from a micro electrode [10].

The evolution into the world of nanomaterials came up after the scientific discovery

of conducting polymers in 1970’s. The intrinsic conductivity in organic polymers,

based on the utilization of the л-electrons, or electron donors or acceptors for charge

propagation in the polymer matrix is significantly improved when nanostructurised

materials are used [7]. Many polymers have been used as matrix materials in carbon

nanotubes (CNTs) /polymer composites for various target applications. Carbon

nanotubes (CNTs) are molecular scale wires with high electrical conductivity,

extremely high mechanical strength, and can be divided into two categories: single-

walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs)

[12]. They resemble rolled up graphite and it was found to act as conductors or

semiconductors with very high tensile strength. The existence of nanotubes within

nanotubes led to the distinction between multi-walled nanotubes (MWNTs) and

single-walled nanotubes (SWNTs), which have been reported for potential use in

photovoltaic devices [13]. Apart from its remarkable tensile strength, nanotubes

exhibit varying electrical properties (depending on the way the nanostructures spirals

around the tube, and other factors, such as doping), and can be superconducting,

insulating, semi conducting or conducting (metallic). Usually, the MWNTs have

larger diameters and better electrical properties than the SWNTs [14].

Http://www.nanotech.now/nanotube reported that ''one dimensional fiber (nanotube)

exhibit electrical conductivity as high as copper, thermal conductivity as high as

diamond and strength of 100 times greater than steel''.

Present day material science and chip technology deals with microstructures of

characteristic morphology with dimensions of 1 and 100 nm [13]. In building up these

27

nanostructures, scientist aims at structuring molecules through specialized synthesis to

build functional superstructures by self-organization [13]. Inorganic, organic as well

as biological materials can be integrated to functionalized systems to build up

nanostructures of novel application in catalysis, electro-optics, high performance

ceramics as well as chemo and gene therapy. The size dependent changes in the

physical and chemical properties of the nanomaterial composite obtained makes them

different from those of the bulk material. The larger surface to volume ratio provides

substantial changes in the chemical, electrical and optical properties. These

developments have produced significant improvements in the areas of electronics,

telecommunication and health care. Recently, the electrochemistry related aspect of

nanostructurised conducting polymers have attracted a great deal of interest because

of their potential application as electric energy storage system, chemical-to-electric or

vice versa energy conversions, sensors and biosensors, and materials for corrosion

protection [8, 10].

2. 1 Preparation methods for conducting polymers of PANi, PPy and PTh

Of the intrinsically conducting polymers, polyaniline, polypyrrole and polythiophene

have been greatly exploited for practical applications in many areas of human needs.

Other electronic polymers are poly-acetylene, poly-(para-phenylene), poly-

(phenylenevinyl-ene), polyfuran, polyindole, polycarbazole and other poly-

(heteroaromatic vinylenes). The structural feature common to conducting polymers is

their alternating single and double bond lattice structure which allows for the transfer

of charge carriers upon excitation by the use of appropriate dopants. Doping materials

can include iodine, bromine, lithium, sodium, mineral acids and surfactants. The

conjugated system of PANi, PPy and PTh is earlier described in Fig. 1.1.

Many publications have appeared on the preparation and blending techniques for

these polymers and their substituted derivatives [15, 16, 17]. Improved solvent

solubility and thermal stability for the polymers were obtained when blended with

conventional polymers such as polyvinyl acetate (PVAc), polystyrene (PS) and

polyvinyl acetate (PVA). However, the conductivity of the blends formed was lower

than the conventional host polymers [18]. Over the years, conducting polymers are

prepared by a variety of techniques including cationic, anionic, radical chain growth,

28

co-ordination step growth polymerization or electrochemical polymerization [17–34].

The electrochemical polymerization of conducting polymers is generally achieved by

galvanostatic, potentiostatic or potential scanning voltammetry. The conductivity and

yield of the polymeric product are enhanced by doping, use of oxidizing or reducing

agent or a protonic acid to give a highly delocalized polycation or polyanion [18].

The preparation techniques aim at getting a product with excellent conductivity and

good environmental stability. The applications of these polymers have suffered

greatly because of their intractable nature and insolubility in common solvents. This

limitation in post-synthesis processability is due to the chain stiffness and inter-chain

interactions that render these materials insoluble in common solvents. The resulting

polymers are usually cross-linked, highly branched, or electrostatically cross-linked

due to polaron/bipolaron charge interactions. This chemical or ionic cross-linking is

the cause of the polymer’s intractability [35]. To curtail this intractable problem

which hinders processability, researchers have fashioned out some preparation

procedures for synthesis of conjugated polymers that are soluble in either or both

aqueous and organic solvents for a target use. Some of the approaches that have been

employed include:

Reduction of polymer to the non-conducting state so as to remove inter-chain

charge interactions. For example, PANi when in the deprotonated form,

emeraldine base (EB), it is readily soluble in some solvents such as N-

methylpyrrolidinone, dimethyl formamide and dimethyl sulfoxide [36].

Use of alkyl substituted monomer for the polymerisation of PANi, PPy and

PTh produces polymers which are rendered soluble in common organic

solvents (chloroform, toluene, etc.) through reduced interchain interactions

and favourable substituent–solvent interactions. Examples include the

synthesis of soluble and conducting poly (3-alkylthiophene) [37, 38] and the

introduction of N-substituent [39, 40] or alkoxy substituent [41, 42] on

Polyaniline. This approach however poses some steric hindrance on the

polymer with a resulting effect of lower conductivity.

Use of large protonic acid with counter ion that is soluble in organic solvents

such as camphor sulphonic or dodecyl benzene sulphonic acid during

synthesis of these polymers [43, 44]. The most successful approach for

increasing the solubility of conductive polymers in aqueous solution is ‘self-

29

doping’. This involves use of ionizable functional groups that form negatively

charged sites in solution which are invariably attached to the polymer chain to

make the polymer conducting. This approach avoids the use of many organic

solvents which have many environmental restrictions. This approach have

been found to produce conducting polymers which are environmental friendly,

good water solubility and good electroactivity and conductivity over a wider

pH range (in the case of polyaniline), and also good thermal stability.

According to http://www.wikipatents.com/5002700.html review, different

permanently doped PANi have been invented and patented using electrically

polymerized polyaniline in covalent combination with an organic dopant

having at least one sulphonic acid functional group (United States Patent

50002700) [45].

Other approaches of producing soluble polymers includes in-situ polymerisation with

a monomer/oxidant mixture involving a chemical oxidizing agent with a formal

potential which is slightly lower than the oxidation potential of the monomer. This

results in a metastable reaction mixture that polymerizes upon solvent evaporation

[46, 47]. Use of template guided enzymatic approach in strong acid polyelectrolyte

e.g. polystyrene sulfonate also provides a lower local pH environment for the

formation of soluble conducting polyaniline [48], polypyrrole [49] and polythiophene

[50]. Using horseradish peroxidase (HRP), Nabib and Entezami (2004) prepared

polypyrrole in the presence of sulphonated polystyrene, as a polyanionic template

[49]. The HRP is an effective catalyst for the oxidative polymerization of pyrrole in

the presence of hydrogen peroxide at room temperature. The reaction is sensitive to

solution pH and it is performed in pH 2 aqueous solutions. Polymerization of pyrrole

by this biological route produced a conducting water-soluble polypyrrole for the first

time. Alternatively, a colloidal dispersion of the polyelectrolyte, polymeric stabilisers

and anionic surfactants could be used [51, 52]. Simmons et al. reported for the first

time the synthesis of colloidal polypyrrole particles using reactive polymeric

stabilizers of poly (2-(dimethylamino)ethyl methacrylate-stat-3-vinylthiophenes) in

1995 [53]. Since then different efforts have been made in the utilisation of other

organic based stabilisers.

30

2.2 Strategies for production of ‘nanostructurised conducting

polymers/Polymer composites’

The drive for modern technologies and new materials in the past decade aroused

movement from the erstwhile conducting polymers to generation of nanostructurised

materials with enhanced conductivity for many areas of technology. It is therefore

important that special materials and technologies should be employed that will generate

the requisite metallization in the new material or target application. The attainment of

this objective demands that both bifunctional additives and reaction conditions that will

enhance the efficiency of oxidative polymerization should be employed. The cationic

radicals and other reactive species, after oxidation forms oligomers, which agglomerate

depending on the synthesis conditions to form nano, micro or macrostructures [8].

According to the 2005 Winner of the IUPAC Prize for young Chemists, Jiaxing Huang,

the nanofibres that form the basic unit for polyaniline is naturally formed at the early

stage of the polymerization reaction and are much smaller in diameter than most

templated or electrospun fibres [54]. The nanofibrilar morphology does not require any

template or surfactant, and appears to be intrinsic to polyaniline synthesized in water.

Two approaches; namely interfacial polymerization and rapidly-mixed reactions were

developed to prepare pure nanofibers. The trick was to suppress the secondary growth

that leads to agglomerated particles. Different dopant acid was used to tune the

diameters of the nanofibers between about 30 and 120 nm while changing the organic

solvent in interfacial polymerization reactions has little effect on the product [54].

There are three broad strategies for obtaining nanostructured-conducting polymers:

Template-free approach: The growth of nanostructured conducting

polymers could be ordered through template-less synthesis by choosing

optimum conditions of electrosynthesis at simple chemically inert

electrodes or chemically via optimized and well specified synthesis

conditions. It is well known that the nature of the anion present in the

electropolymerization solution determines the morphology and properties

of the generated conducting polymers. In their conducting form, these

polymers are usually proton and anion doped, whereas reversible

31

expulsion and bonding of anions occurs during reduction and oxidation

processes. It has been shown recently that the presence of suitable dopant

anions leads to the formation of well defined nanostructures of conducting

polymers. Tubules of polypyrrole with diameter range 0.8 – 2.0 µm and

length of 5 – 30 µm were synthesized by the electrochemical template-free

technique in the presence of β-naphthalene sulphonic acid (dopant) and

stainless steel electrode [55]. Shortly after this, polypyrrole nanotubules

with diameter ranging from 50 nm to 2 µm were synthesized

electrochemically in the presence of β-naphthalene sulphonic acid or p-

toluene sulphonic acid as dopant [19]. The micelles of dopant or dopant-

monomer clusters act like a template to orchestrate tubular growth of the

polypyrrole. Using this approach but varying the chemical synthesis

conditions, various nanostructures of modified polypyrrole doped with β-

naphthalene sulphonic acid with particle diameters of 20 – 3000 nm were

reported recently [56, 57].

Template assisted: This approach involves use of an electrically insulating

template possessing nanosized pores. Two common architectures are

employed, one is the track-etched polymer membranes and the other is the

alumina template which is used to house the nanosized structures of

conducting polymers [8, 22]. Track-etched template was used by Pra et al.

to prepare electrochemically assembled copper nanowires and polypyrrole

nanotubules [58]. Demoustier and Stavaux used nanoporous polycarbonate

track-etched membrane to prepare polypyrrole doped with poly (sodium 4-

styrenesulfonate) with thickness that is dependent on the pore sizes and

type of electrolyte used [8]. Duvail et al. reported the electrochemical

polymerization of poly (3, 4-ethylenedioxythiophene) within the pores of a

track-etched porous membranes [23]. Polymers generated using this

technique is associated with increased electric conductivity because the

polymerization is confined to the pore spaces and electrostatic interactions

between the ionic species allow alignment on the walls of the pores [8].

Alumina template is very popular in the synthesis of conducting polymers

and copolymers. Polypyrrole nanowires was electro synthesised by direct

oxidation of pyrrole in a medium of 75% isopropyl alcohol + 20% boron

trifluoride diethyl etherate + 5% poly (ethylene glycol) (by volume) using

32

porous alumina membranes as the templates. The as-prepared nanowires is

composed of smooth surface, uniform diameter, and highly ordered tip

array. The SEM and TEM images of the nanowires are shown in Fig. 2.1.

The conductivity of polypyrrole wires is better than that from flat films

and a single polypyrrole nanowire was measured to be 23.4 S cm−1 at room

temperature [59]. The field emission properties of the polypyrrole

nanowire arrays prove positive for its use as emission device nanotips.

Figure 2.1: SEM (a–c) images and TEM image (d) of PPy nanowires [59].

Molecular template assisted electrosynthesis. This approach is similar to

the template-free method described earlier. The major difference is that the

electrode is physically modified with specialized kind of adsorbate that

directs electropolymerisation to proceed in template-like manner.

Nanosized fibres of polypyrrole films have been grown galvanostatically

33

in acetonitrile solution at a gold electrode modified with thiolated β-

cyclodextrin self-assembled monolayer [60].

Other approaches that have been used to make nanostructured conducting polymer

composites are the chemical or electrochemical reaction of nanosized metal ions from

their salt solution at the polymer/solution interface [8]. The high metal surface area of

the generated polymer composite confers a higher degree of electrocatalysis on the

metal-conducting polymer nanocomposite. Different polyaniline fibre with gold

nanoparticles, have been demonstrated for use as memory devices for data storage

(e.g. flash memory) [54]. High catalytic nano-sized gold-polypyrrole complex was

electrochemically prepared from gold complexes of Au(ClO4)4- with less than 100 nm

particle sizes have been reported [21]. Use of nano-sized inorganic materials such

V2O5, TiO2, Fe2O3 have also been reported to produce metal-oxide based polymer

composites [8]. Clay-based nanocomposites have also been explored under this

category. Some organic materials such as Nafion membranes, DNA and porphyrin

derivatives have been used to produce nanocomposites of polypyrrole and

polythiophenes for different applications [8].

2.3 Polyaniline (PANi) blends/composites

The polyaniline’s is a very important class of ICP with varieties based on the type of

substituent on the aniline monomer. The synthesis of the conducting polymer “aniline

black” was first published in the 19th century as a product of the anodic oxidation of

aniline, but its electronic properties was not established for many years [61]. PANi is

novel due to its relative facile processability, electrical conductivity and

environmental stability [62, 17]. In acid solution, it is protonated to form an organic

metal, which is easily deprotonated in alkaline medium to form a semiconductor. It

generates a conductivity of about 105S/cm comparable to copper. These inherent

advantages make PANi to qualify as a synthetic metal. PANi exists in three well-

defined oxidation states: the electrically insulating leucomeraldine (LM), electrically

conducting emeraldine (EM) and the electrically insulating pernigraniline (PE) as

shown in Fig. 2.2.

34

Figure 2.2: Schematic diagram of PANi in different oxidation states namely, LM

(insulator) as (A), EM-base (insulator) as (B) and PE (insulating) as (C).

The LM and PE states are the fully reduced (i.e. all the nitrogen atoms are amine) and

the fully oxidized (all the nitrogen atoms are imines) forms respectively. In the EM

state, the ratio of amine to imine is 0.5. This explains the adopted general formula:

[(-B-NH-B-NH) y (-B-N=Q=N-) 1-y] x showing PANi as a continuum of oxidation states. ‘B’ and ‘Q’ denote benzene rings

in the benzoid and quinoid forms respectively. Letter 'y' is a measure of amine to

imine forms; when y = 1, it implies PANi in the Leucomeraldine form, y = 0.5 implies

PANi in the Emeraldine form and y = 0 implies PANi in the Pernigraniline form. The

imine’s sites of the half oxidized EM base form are easily protonated in strong acid

solutions to produce the conductive EM salt form of PANi [62].

Strafstrom et al. was reported to have proposed the polaronic lattice structure for the

EM salt showing the positive charges without any change in the number of pi-

electrons. The combination of charged sites and electro neutral radicals (i.e. solitons)

form polarons; at higher doping levels, two polarons could combine to form

bipolarons [63]. In the review work of Gospodinova and Terlemezyan, the mechanism

of the oxidative polymerization of PANi was reported and discussed [63]. The

35

chemistry of the dopant used and the polymerization mechanism determines the

properties, morphology, and molecular weight distribution of the PANi product.

In the last two decades, a lot of effort has been made on the search for improved and

processable PANi having good mechanical and solubility properties. This involves

preparation of conducting PANi blends and composites, which possess the

mechanical properties of an insulating host matrix and also the electrical properties of

the PANi Guest within the matrix. Polymeric composites of conducting polymers

offers the combination of the good processability of a polymer like poly-

(phenylenesulfide-phenylenediamine) PPDA with the conductivity and ease of

synthesis of another polymer like PANi [62].

Two general methods are being adopted, one was to introduce flexible substituent to

the benzene rings or nitrogen atoms of PANi [64]; the other is doping PANi with

functionalised organic acid such as camphor sulphonic acid (CSA), naphthalene

sulphonic acid (NSA) and dodecyl benzene sulphonic acid (DBSA) [24, 25, 26, 27].

These processes produce a soluble and more conductive product that could be blended

with PANi or used to form PANi composite with materials such as poly vinyl alcohol

(PVA) or polyurethane [28]. In the solid phase, the protonation of the imine sites of

PANi or its blend/composites is limited by the diffusion of the dopant (acid) [17].

This also depends on the dopant anion size and the polymer matrix morphology. The

more the degree of dispersion of dopants, the more the redistribution of polarons

along PANi macromolecular chains and the more conductive the PANi blend or

composite. Over the years, the following broad synthetic methods have been

employed in preparing PANi blends or composites:

Polymerization of aniline in dispersion systems involving the use of

dopants, surfactants and suitable oxidant [17, 25, 26, 27].

Chemical or electrolytic in situ polymerization of aniline in the presence of

a polymer matrix or by incorporation of polymer/metal nanocomposites

like H2PtCl6 at a liquid liquid interface [6, 7, 17, 34]. This method could׀

involve use of reverse micelles, during which polymerisable surfactants

were used or incorporation of metal nanoparticles that could enhance the

conductivity of polymers. The principle is based on the reduction of metal

ions (clusters) that are dispersed in polymer matrices, or the

36

polymerization of the monomer dispersed with the metal nanoparticles

followed by electrodeposition of the strongly embedded metal / polymer

composites by either pulse or potential cycle program [34].

Use of electrolytic method to polymerize aniline in suitable electrolyte at

an electrode (anode) [17, 61, 62, 65, 66]. The conductive form of

polyaniline was synthesized by the anodic and chemical oxidation of

aniline in malonic acid medium [66]. The conductivity of polyaniline

doped with malonic acid changed from 1.62×10–6 to 2.5×10–5Scm–1

depending on the way it was synthesized. The polymer growth rate was

observed to be very slow in malonic acid compared with H2SO4.

According to Li et al. [62], electropolymerised PANi has many advantages

over the chemically polymerized PANi. These includes better adhesion

and good mechanical strength; ease of control of film thickness based on

the applied potential and number of cycles to give a more stable electronic

property for a longer duration. Also the product is purer as there are no

residual oxidants or reduced specie adhering to the polymer matrix thus

providing a more ordered nanostructure.

Grafting of the polymer on a PANi surface or copolymerized with another

polymer surface [17, 67, 68]. This offers production of polymeric systems

that are copolymers, composites, bilayers or blends with improved

properties of the corresponding homopolymer. Free standing-grafted

polymer films of PPy and PANi prepared electrolytically showed varying

amounts of polyaniline and polypyrrole depending on the electrolysis time.

The Raman spectra of the films taken from the electrode side were similar

to those of pure polyaniline, whereas the spectra of the solution side were

identical to those of pure polypyrrole [67].

2.4 Polypyrrole blends/composites

Polypyrrole (PPy), known as “polypyrrole black” since early 20th century, is one of

the most studied COPs. PPy is an extensively studied CP with its monomer pyrrole

being easily oxidisable, water soluble, commercially available and the PPy and its

derivatives possess good environmental stability, good redox properties, high

37

electrical conductivity and simplicity of synthetic procedures. The main hindrance of

its processability is in its insolubility in any organic solvents. PPy is usually a black

powder and could be prepared by chemical or electrochemical synthesis. Various

approaches have been used for the chemical synthesis of polypyrrole nanofibres and

nanaotubes such as soft (organic) and hard (inorganic) template approach and of

recent nanofibre seeding, in which narrow pore nanotubes with an average inner

diameter of 6 nm and an outer diameter of 60 – 80 nm respectively are formed [69,

70]. The new approach involved the use of reactive seed templates that chemically

react with the monomer prior to the addition of oxidant. This pre-polymerization

reaction on the surface of fibrilar seed templates helps direct the evolution of bulk

fibrilar morphology when oxidant is subsequently added.

The mechanism of polymerization for both chemical and electrochemical methods

follows an initial generation of radical cation while subsequent steps depend on the

type of polymerization being adopted. In the chemical case, the radical cation attacks

another monomer to form a dimer radical cation, while in the electrochemical synthesis

a neutral dimer is formed by the dication of the large concentration of radical cations in

the vicinity of the electrodes. The general scheme for the formation of the dimers and

subsequent chain growth for both methods is shown in Fig. 2.3. The number of

electrons involved in the polymerization process for each monomer is approximately

2.25, one for each of the two α positions to form the polymer and another one for every

four monomer units to form the doped polymer. Similarly, one anion per four monomer

units is incorporated during doping to maintain charge neutrality [71]. The chemical

synthesis is used when large quantities of material are required but electrochemical

synthesis is preferred for research purposes due to the simplicity of the technique,

control over material thickness, geometry and location, the facility for doping during

synthesis, the wide choice of available dopant ions and the generation of good quality.

The electro-deposition of polypyrrole on the positively polarized working electrode

proceeds via a condensation reaction between the pyrrole-monomer units, and the

concomitant balance of charge (electroneutrality) along the polypyrrole backbone by

the anions present in solution [32].

38

Figure 2.3: Generalized scheme for the polymerization of polypyrrole from

monomer (a) and dimer (b) [71].

Figure 2.4 shows the scheme for the electropolymerisation of polypyrrole which is

typical for other aromatic five-membered heterocycles with NH, S and O as the

heteroatom. Monomer units are adsorbed onto the surface of the working electrode

resulting in one-electron oxidation to form a pyrrole radical cation. These radical

cations then couple with themselves, or with other cations or with neutral monomers

from solution. In each case, this leads to the formation of a dimer dication, which

undergoes a double deprotonation to give a neutral molecule. These more stable dimer

radicals have a lower oxidation potential compared with the monomer units and chain

growth then occurs by preferential coupling between the dimers and monomers [32,

71]. Anions called ‘dopants’ are incorporated into the film to maintain electrical

neutrality as polymerisation progresses.

39

Figure 2.4: Scheme for the electropolymerisation of polypyrrole [32].

In the undoped state (pristine), neutral PPy is generally non-conducting having the

aromatic and quinoid structures of which the latter possesses a higher energy

configuration. In the presence of a Lewis acid, the doped form (oxidized) is formed

which manifest as either a polaron and/or bipolaron depending on the doping level as

follows:

PPy + X-(s) → PPy+X- + e- polaron

PPy+ + 2X(s) → PPy2+(X-)2 + e- bipolaron

40

where X- are anions, (the subscript (s) indicates that they are in solution phase), and PPy

represents a segment of the polypyrrole chain.

The various structures of PPy are shown in Fig. 2.5. The chemical oxidations of

pyrrole monomer using aqueous or anhydrous iron III chloride (FeCl3), other salts of

iron (III) and copper (II) and other Lewis acids and halogens have been reviewed

[71]. The yield from the reactions is dependent on the choice of solvents and oxidants,

initial monomer/oxidant ratio, duration and temperature of the reaction.

Figure 2.5: Scheme for the structures of polypyrrole showing the non-

degenerate: aromatic (a) and quinoid (b) configurations; and degenerate

(oxidised forms): a polaron defect (c), and a bipolaron defect (d) configurations.

A yield of almost 100% is obtained with Fe (III)/monomer ratio of 2.4 [71]. The use

of different concentrations of dopant materials has also been employed to modulate

the yield and morphology of polypyrrole in aqueous medium [56]. Details of the

various micro/nanotubes reported in [56] using different dopant/monomer (d/m) ratios

and temperature settings are discussed further in Chapter four.

FeCl3 have been used to chemically synthesis PPy in various solvents (water,

alcohols, benzene, tetrahydrofuran, chloroform, acetone, acetonitrile and dimethyl-

41

formamide) [72]. The highest conductivity was obtained from PPy prepared in

methanol solution [71]. Rapi et al. [72] reported improved chemical yield using

shorter times of polymerization and lower temperatures (0 – 5 ºC). Improved film or

coatings of PPy was prepared through the use of gaseous pyrrole on FeCl3. Persulfates

(Na2S2O8,), (NH4)2S2O8) and H2O2 have also been applied as oxidant in the synthesis

of PPy [71 - 73]. The use of peroxide as oxidant can be via a hydrogen peroxide based

chemical or biological material. Recently, self-assembled conducting PPy

micro/nanotubes and films using β-naphthalene sulphonic acid were chemically and

electrochemically prepared [19, 55]. Micellic clusters of the dopant of NSA and its

complex with the pyrrole served as template for this tubular growth.

Composites of PPy with different organic and inorganic compounds having improved

mechanical performance have also been reported. Omastova et al. prepared net-work

like PPy composites with poly (methyl methacrylate (PMMA/PPy) [74] while Cairns

et al. studied the structure of PPy composite with polystyrene and reported that

pyrrole was present as discrete 20 – 30 nm nanoparticles adsorbed into the PST matrix

[75]. Lu et al. [76] chemically synthesized and characterized nanometer sized PPy

composites using styrene and butyl acrylate. Also, polypyrrole/poly (methyl

methacrylate) coaxial nanocable was prepared through the sequential polymerization

of methyl methacrylate and pyrrole monomers inside the channels of mesoporous

silica template, followed by the removal of the silica template [77]. Similarly,

thermoplastic polyurethane (TPU- PPy) composites on electrode [78] and PPy- poly

(vinyl alcohol or chloride) blends [79] have been reported.

Although PPy can be chemically synthesized especially for large-scale preparation,

electropolymerisation is easily achieved and is the most common preparative method

because of the ease of oxidizing the polypyrrole compared to the pyrrole monomer.

Electropolymerisation could be achieved with different electrodes such as platinum,

carbon and metal oxides. The earliest electropolymerised film was prepared on

platinum electrode by the oxidation of the monomer in sulphuric acid solutions. The

polymer obtained was found to be brittle, conductivity of 8 Scm-1 and one positive

charge per three pyrrole monomer units. Subsequently, improved electrochemically

synthesized PPy have been prepared by variations in conditions of preparation

42

(solvent, temperature, anions, current/potential) used and on the composition of the

solution used during electrochemical investigations [80-87].

Tamm et al. [80, 81] and Yuan et al. [86] had respectively reported an elaborate in-

situ electrochemical study on the redox properties of polypyrrole in aqueous solutions

using different anions and polymerization conditions to show the concepts of

electroneutrality coupling and electron hopping which are useful for the

fuctionalization of the polymer and its electron transportation ability. They found out

that the polymer differs with respect to the extent of oxidation, doping anions, and

also reduction, doping cations. Even though polymerization efficiency improves as

the positive potential limit becomes more positive, however, polypyrrole films

becomes over-oxidized at potentials in excess of 700 mV versus Ag/AgCl [86]. The

overoxidation, which is entirely irreversible, results in the dedoping of conducting

polypyrrole and the formation of carboxyl group. Overoxidation of PPy, leads to loss

of conductivity and de-doping.

Study on the potentials at which the overoxidation of polypyrrole occurs have shown

a strong dependence on the pH values of the aqueous solutions [87]. At higher pH

values, the occurrence of the overoxidation was easier, thus occurring at lower

overoxidation potential. This is coincident with the electrolysis reaction of water to

give out oxygen. It was supposed that the oxygen produced in the electrolysis of water

results in the overoxidation of the conducting polypyrrole films.

The polypyrrole redox scheme (Figure 2.6) is accompanied with three processes,

namely, mass and resistance changes as well as electron transitions unlike other

systems in electrochemistry in which only electrons are involved during the reduction

and oxidation processes [88, 89]. Using cyclic voltammetry, oxidation of pyrrole

yields a positive charged polymer film on an electrode surface, which is balanced by

anions incorporated during synthesis. During the following reduction scan,

electroneutrality can be maintained either by expulsion of these anions or by

incorporation of cations. When a sufficient negative potential is applied to the

polymer, the anions are expelled (undoping), thus reducing it to the neutral state.

43

NH n

+

A-

NH n

0

+ A-+e-

-e-

Figure 2.6: The polypyrrole oxidation and reduction scheme

Conversely, when a positive potential is applied to oxidize the neutral film (doping),

the anions are taken up. Therefore, the reduction/oxidation of PPy films involves the

transport of ions in and out of the polymer in order to compensate the cationic nature

of oxidized polymer. The dopant anions (small size) in the PPy film can be exchanged

relatively easily with electrolyte anions in aqueous solutions during potential

sweeping. The anion exchange processes offer an alternative chemical route for the

preparation of some conductive and anion specific PPy complexes. However, the

anion exchange processes do not result in any significant alteration of the basic

structure of the polymer.

In the investigation of the effect of solution temperature on electroactivity of

polypyrrole using cyclic voltammetry technique by Khalkhali & Wallace, thermal

treatment at elevated temperatures in solutions was found to affect the electroactivity

of PPy, and the changes are mostly irreversible [89]. The temperature, time of

exposure, the switching potential and the nature of electrolyte in which the polymer is

switched all have important effects in determining thermal stability of the polymers in

solutions. The nature of the effect was greatly dependent on the nature of the counter-

ion incorporated during synthesis. Large surfactants and immobilized polymeric

dopants such as polyelectrolytes counterions were not released during reduction

process and become trapped within the polymer matrix due to their large size and

entanglement with the PPy chain; this consequently increased their solution stability

at elevated temperatures. Smaller inorganic ions dopants such as chlorides gave a less

stable polymer. The loss of electroactivity of PPy films when exposed to elevated

solution temperatures in aqueous media was attributed to the loss of conjugation in

polymer due to the nucleophilic attack by water or dissolved oxygen, which occurs

more rapidly at higher temperatures [89].

44

2.5 Polythiophene blends and composites

The property of PTh shows much resemblance to those of PPy. The monomer,

thiophene is quite cheaper than pyrrole and its redox potential is quite higher. It shows

remarkable stability in both air and water. PTh usually appears as a stable infusible

black powder, which is insoluble in common solvents. The chemistry of PTh is based

on its conjugated π electrons. Its conductivities in the neutral (undoped) form could be

as low as 10-11 Scm-1 while in the oxidized (p-doped form) it could be as high as 102

Scm-1, although on exposure to air this value decreases [90]. The

electropolymerisation of most conductive polythiophenes is done in non-aqueous

solvents [30]. Unsubstituted thiophene is insoluble in water and can thus not be

polymerized in aqueous medium. This is due to the electro-oxidation potential of

thiophene, which is higher than the decomposition potential of water coupled with the

fact that the reaction between thiophene cation with water prevents cationic

polymerization and this becomes a competing reaction [30]. The electrosynthesis of

PTh could be done galvanostatically, potentiostatically or by sweeping potential

voltammetry [29, 30, 33]. P-doped PTh has a characteristic blue colour, highly air

sensitive and its conductivity decreases on exposure to the atmosphere. N-doping is

less studied but cathodically doped PTh have been reported with lower conductivity

as compared to the p-doped polythiophene [91]. The processes of p- and n- doping in

PTh compounds and their effect on conductivity have been described by the

‘generalized transmission line circuit’ model using electrochemical impedance

spectroscopy [91, 92].

The mechanism of polymerization of PTh and many substituted PTh’s is similar to

those of PPy discussed earlier occurring via a radical – monomer coupling or radical –

radical coupling intermediate to form the polymer [90]. In the process, radical cations

(polarons), or dications (bipolarons) are formed. Substituted thiophene polymerizes at

1.7 V vs. SCE. This value is lower than the oxidation potential of the corresponding

polymer and thus over-oxidation readily sets in which causes degradation of the

polymeric film. The problem of over-oxidation is usually minimized in different ways

[30]. This could be done by the addition of bithiophene to the thiophene solutions or

converting the thiophene to its oligomers before electrolysis. The resulting extension

of the conjugation length and lowering of the oxidation potential produces a polymer

45

with high conductivity and good electrochemical and thermal stability. Cho et al. have

reported that substitution of the hydrogen on the monomer with electron donating

groups do significantly reduce the potential at which oxidation occurs [33]. The

longer the length of this substituent, the lower is the oxidation potential of the

monomer. Various substituents such as long alky chain, sulfonate groups, carboxylic

acids, amino acids and urethanes have been used to improve the polymers solubility

[29]. The sulfonate substituent group (-SO3H) acts as a counter ion and produces self

doped conducting polymer.

Ballav and Biswas reported a simple procedure for the preparation of PTh in high

yield using FeCl3 as the oxidant and using the same method they were able to prepare

PTh-Al2O3 nanocomposites [93]. Cyclic voltammetric experiments have shown that

the electrode reactions taking place depends on the conditions used, the type and

length of electron donating substituent and type of solvent used [90, 94]. Slowly

polymerized PTh has more conductive states and relatively higher molecular weight

than those from faster chemical reactions [95]. The positive mechanical properties

exhibited by PTh over PPy coupled with its larger positive working window makes

copolymers of PPy and PTh as potential better positive electrodes in lithium batteries.

Yigit et al. [96] electrochemically prepared composites of PTh with natural and

synthetic rubber as the insulating polymer host. Various regio-regular PTh’s with

pendant carboxylic acid functionality have been prepared as water soluble conducting

polymer which produced distinct color changes when applied on divalent cations and

some acting as solid state chemo sensors for acid vapors [97]. It appears that PTh

films give more electro catalytic response even though the preparation procedure

appears more cumbersome and must be done in totally anhydrous solvent to get good

reproducibility. However, a wider application of PANi and PPy is practiced.

2.6 General properties and factors affecting processability of nanostructured

polyaniline, polypyrroles and polythiophenes.

The properties and stability of nanomaterials determines the final application to which

the conducting polymer is put. Some of the intrinsic properties that characterize NCPs

46

do originate from the preparation procedure. Understanding of the chemistry,

electrochemistry, structure, electrical and optical phenomena, processing and

applications of NCPs has been employed in improving the processability limitations

of PANi, PPy and PTh. The effect of temperature and ageing process, nature of

solvent, pH and redox potential of the polymerization liquor, and the nature of

dopants on their preparation and stability are discussed below.

2.6.1 Temperature and ageing process

Zhang et al. [27] reported the temperature dependence of conductivity for PANi

doped with various types of NSA showing decrease in conductivity with decrease in

temperature, thus exhibiting semiconductor behavior. Kassim et al. [98] studied the

effect of preparation temperature on conductivity of PPy films prepared by

electrochemical synthesis in aqueous medium using camphor sulphonates as the

dopant and temperature range of 1 – 60ºC. It was discovered that the film’s

conductivity decreases with increase in temperature, the optimum conductivity was

obtained between 10 - 30ºC. They established that PPy formed at low temperature is

stronger and more conductive than those formed at higher temperatures. SEM analysis

for the PPy films prepared at lower temperature attested to its smoother, more

coherent, and stronger mechanical strength when compared with that prepared at

higher temperatures. The X- ray analysis also showed that the PPy film obtained by

low temperature preparation is more compact with higher conductivity. Many

researchers have established that the polymerization of thiophene and its derivatives is

temperature dependent. The highest quality material was obtainable at 5ºC, with rate

increasing up to 16ºC after which there is decrease [90].

2.6.2 Nature of solvent

Many researchers have shown that the electrochemical behavior of a polymer is

determined by the nature of the electrolyte used for polymerization [99-103].

Monomers with lower oxidation potentials (aniline and pyrrole) can be polymerized

in aqueous electrolytes. For monomers with higher oxidation potentials such as

thiophene, aprotic solvents like acetonitrile, propylene carbonate, dichloromethane

etc. must be used. This is why the aqueous synthesis of polythiophene is highly

47

problematic. The use of acidic solvents for polymerization of PTh have been reported

to substantially lower the oxidation potential of the monomer from about 1.6 V to 0.9

V vs. SCE [99]. The solvent composition used in the preparation of a conducting

polymer alongside the film thickness and morphology have been reported to

determine the biosensing efficiency [100].

2.6.3 pH and redox potential of the environment

The electronic conductivity of conducting polymers changes over several orders of

magnitude in response to changes in pH and redox potential of their environment

[104]. Unlike aniline and pyrrole, which could be electropolymerised at low potential,

thiophene requires much higher oxidation potential [90, 94]. The effect of pH,

oxidation potential and nature of electrolyte on conductivity of PPy films synthesized

therein have been extensively studied and reviewed [104]. In alkaline media, thin

insulating films are formed [105], while optimum conductivity was obtained around

the synthesis potential of 0.6V (vs. saturated calomel electrode) in aqueous solutions

[106]. The absence of a protonic dopant ion during electropolymerisation reaction of

conducting polymers do limit the rate determining step of proton abstraction from the

monomer molecule (H-M-H), and thus terminating the polymerization at the dimer or

trimer stage. A reasonable level of doping is thus critical for the formation of higher

oligomers. Lower oligomers are soluble in nucleophilic environment just as the

monomer, but with increasing polymerization an insoluble film is formed at the

electrode. This trend increases from aniline to pyrrole and to thiophene [107, 108].

2.6.4 Nature of dopants

The solubility and conductivity of the polymer could be tuned by the nature and

concentration of the dopant. Use of specialized dopants during polymerization plays a

major role in tuning electronic properties in conducting polymers. It helps to create a

more stable polymer whose work function (work of extraction of an electron from an

uncharged metal) and resistivity can be electrochemically adjusted [15, 107]. While a

particular dopant may produce nanostructurised polymer from a particular monomer,

it may form completely different morphology when another monomer is used.

Nanofibres of PANi which could readily be produced using the surfactant-mediated

48

synthesis with naphthalene sulphonic acid become greatly problematic with PPy.

Rather than nanofibres, different fibrilar and tubular morphology with diameters of 50

– 2000 nm range are formed [19, 55]. However, with different naphthalene sulphonic

acid derivatives and depending on the dopant structure and concentration, nanotubes

of PPy with average diameter of 130 to 250nm could be produced [27, 73]. Similarly,

the polymeric yield of conducting polymers is greatly influenced by the relative

concentration of the dopant used [15, 56, 57, 95].

2.6.5 Other factors

Amongst other factors that could affect the stability of conducting polymers is the

nature of the working electrode. This determines its relative catalytic property and

lifespan before fouling sets in. Del-Valle et al. have reported on the comparative

performance of modified electrodes using PANi, PPy, and PTh in dispersion system

of Pt or Pt – Pb respectively to monitor the catalytic oxidation of formic acid [109].

The modified system of PTh had the longest electro catalytic activity while systems

with PANi and PPy were fouled within a shorter time. This informed their

recommendation of the use of PTh as a better electro catalyst in fuel cells and sensors.

2.7 Characterization of nanostructured PANi, PPy’s and PTh’s

The use of physical and chemical methods for the characterization of NCPs is critical

for the confirmation of the electronic species involved vis-à-vis the understanding of

the electrochemical processes taking place and for fashioning procedures for

development of appropriate application for the polymer. The major characterization

techniques often employed are morphology, electrochemical and spectroscopic

examination of the nanotube or polymer film produced depending on the potential

application that is of interest.

2.7.1 Morphology characterization

Scanning electron microscopy (SEM), transmission electron microscopy (TEM),

atomic force microscopy (AFM) and X-ray diffraction measurement (XRD) have

been severally used to characterize polymers of intrinsically conducting polymers and

49

their composites [25, 26, 27, 73, 51, 93, 98] The polymer could have different

morphology ranging from grain-like, sponge-like, fibrous, porous-structured to

tubular (hollow or closed) morphology. SEM and TEM studies of PTP-Al2O3

composite particles showed globular morphology with uniform particle size for

polymers prepared in CHCl3 while those prepared in the absence of any solvent

showed relatively irregular particle formation. The morphology studies of screen-

printed carbon electrode (SPCE) modified with PANi showed sponge-like high

surface area suitable for enzyme immobilization, which changed to a speckled, grainy

image after immobilization. Changing the chemical structure of the dopant and

synthesis conditions leads to formation of different morphologies for PANi/NSA

nanotubes [25, 27]. This varies from hollow to solid micro/nanotubes and fibres

depending on the type of NSA used and oxidant concentrations [56, 57].

Also, Zhang and Manohar reported the synthesis of narrow pore-diameter PPy

nanotubes by the chemical oxidation of pyrrole with FeCl3 as oxidant, and V2O5 as the

sacrificial seed template to orchestrate fibrilar polymer growth [69]. This seed

template has the catalytic ability of oxidizing the monomer prior to the oxidation with

the oxidant. The same authors also reported fibrilar and tubular morphology when

large organic dopant anions such as naphthalene sulphonic acid are used to during the

synthesis. Very thin nanofibres were obtained when the reaction is carried out in

ethanol using FeCl3 as the oxidant and V2O5 as seed template [70]. The SEM of the

as-synthesized polymer nanotube showed that fiber diameter can be controlled by

stirring the seed template in ethanol for a limited time before initiation of the

oxidation of the monomer. Figure 2.7 (A) shows SEM for polypyrrole-Cl nanofibers

of 30 nm diameters obtained by stirring the seed template foe 30 min, (B) is the

nanofiber of 100 nm diameters obtained upon extended stirring in ethanol for 12 h.

There is the need to activate the seed template to ensure fibrillar polymer growth in

polypyrrole whereas some polyaniline nanofibres are readily produced in the

unseeded polyaniline system which was interpreted as a peculiar feature intrinsic to

polyaniline. The similarity or contrast in morphology could further be investigated

using the XRD measurements. PANi indicated four sharp characteristic peaks at 2θ =

4.6º, 9.2º, 13.8º and 23.0º for the crystalline tubes while amorphous PANi had only

two characteristic broad peaks at 2θ = 20º and 26º.

50

Figure 2.7: SEM micrographs of as-synthesized PPy-chloride nanotubes from

ethanol with V2O5 as sacrificial template, where A is morphology obtained after

30 min while B is the morphology after 12 hr of stirring [70].

While PPy/NSA tubes were amorphous, the polypyrrole doped with p- toluene

sulfonate acid (PPy/TsOH) was partially crystalline [19, 55]. XRD is also employed

to discriminate the compactness of a polymer film synthesized at different

temperatures. The peak values for polypyrrole doped with p- toluene sulfonate acid

(PPy/CSA) film were greater at lower temperatures than at higher temperatures [98].

2.7.2 Electrochemical characterization

Various voltammetric techniques are being used to study the redox activity and

electrochemistry of conducting polymers of PPy and PTh tubes and films. While

cyclic voltammetry (CV) is the most versatile for characterization of electrochemical

systems, other techniques such as Oster-Young square wave and differential pulse

voltammetric techniques (OSWV and DPV) have been severally used for both

qualitative and quantitative analyses [68, 91, 92].

51

Polypyrrole prepared from aqueous medium has poor/ill formed oxidation and

reduction peaks. However when it is modified with suitable dopants, the

electroactivity is enhanced. Similarly, electrochemical impedance spectroscopy (EIS)

offers a platform for a deeper intuitive study of interfacial properties of conducting

polymers at electrode surfaces. The changes in the electrochemical parameters during

the course of switching through the oxidation and reduction waves is dependent on

the properties of the polymer, the electrolyte and the external force applied through

the electrode surface.

2.7.3 Spectroscopic characterization

UV-Vis, FTIR and 1H NMR spectroscopic techniques are widely used for the

characterization of NCPs. The UV-Vis absorption spectra are often used to diagnose

the presence of charge carriers within the polymer structure or the existence of

various oxidation state(s) of the NCPs. While the emeraldine salt form of PANi is

characterized with two absorption bands at about 346 nm and 636 nm for the π- π*

excitation band and the polaron bands respectively, the higher doped PANi may have

multiple polarons and bipolarons depending on the synthesis condition, type of dopant

used and the level of doping [25, 26, 34]. The same trend applies to PPy and PTh in

the doped state, however the relative position and intensity of the polaron and bi-

polaron bands changes depending on the synthesis conditions [19, 55].

In the study of Massari et al., the unique light absorbing behavior of PANi at various

redox states under a modulated electrochemical oxidation was explored for illustrating

the interplay of the index of refraction and absorptivity of the material [110]. In

investigating the electrochemistry of polyaniline or polypyrrole synthesized via

surfactant dopant, FTIR spectroscopy is a viable tool to monitor the presence or

absence of surfactants as well as the level of over-oxidation attained at different

electronic states [15, 17]. In-situ study using substractively normalised in-situ Fourier

transform infra-red spectroscopy (SNIFTIR) on the structural changes during

oxidation and reduction of the polymer at different stepping potential will manifest

slight shifts in the vibrational peak positions depending on the extent of doping. The

reproducibility of the peak positions during repeated oxidation and reduction is an

attestation of a polymer’s high electroacitivity and suitability for use in batteries and

actuator applications. The sulphonated polypyrrole spectral region between 1000 and

52

1700 cm-1 contains information on the relative populations of charge defects as a

function of potential, and the results confirm that electronic charge transport at

moderate potentials is due to formation of radical cations, which oxidize further or

recombine to form dications at higher potentials. At more positive potentials the

polymer degrades with formation of carbonyl, hydroxyl groups and also CO2 [15, 17].

The electrochemical overoxidation of conducting polypyrrole nitrate film in aqueous

solutions was studied by a combination of cyclic voltammetry, in-situ spectro-

electrochemical measurement and FTIR spectroscopy [87]. The overoxidation, which

is entirely irreversible, results in the dedoping of conducting polypyrrole and the

formation of carboxyl group. The combination of spectroscopic techniques with

electrochemical and morphological methods is indeed a vital force for characterizing

and probing of structural and kinetic reactivities of nanostructured conducting

polymer systems and significant breakthrough is expected from this within the next

decade.

2.8 Application of NCPs in sensors

The application of NCPs is being greatly exploited in bioelectrochemistry,

bioelectronics and voltammetric chemical sensing. The applications of ‘NCP’ could

be generalized under two major groups; namely those derived from its conductivity

and those derived from its electro activity. There are many publications on the

prospect and usage of ‘NCPs’ based on its conductivity in electrostatic materials,

conducting adhesives, electromagnetic shielding devices, artificial nerves, antistatic

clothing, piezoceramics, diodes, transistors and aircraft structures [17, 61]. Salient

features of conducting polymers and their wide biosensor application in health care,

food industries, environmental monitoring was reviewed by Gerard at al. [111]. The

electroactivity properties equally find applications in molecular electronics,

interdigitated microelectrodes (IME), chemically sensitive field effect transistors

(chemFET), electrical displays, biosensor and chemical sensors, thermal sensors,

rechargeable batteries and solid electrolytes, ion exchange and drug release systems,

optical computers and electromechanical actuators or switches [17, 61, 62, 110, 111,

112].

53

Conducting polymers such as polypyrrole (PPy) is unique in its ability to be used in

tailoring specific bulk and surface properties of materials to provide novel solutions

for tissue engineered systems. This includes controlled cell assembly (micro and

nano-patterned surfaces), drug release (degradable polymers), tissue release (thermo-

responsive polymers) and integrated biosensing (electroactive polymers). In addition,

such materials provide a platform for the study of the fundamental science relating to

tissue-material surface interactions. It is in recognition of these special requirements

that researchers have engaged the use of polypyrrole in particular, for trial use in

biological applications [32]. The electric conductivity of NCPs is sensitive to changes

in chemical environment; this provides possibility of developing various sensors. For

example, spectroelectrochemical studies have shown that the change in absorptivity

with effective index of refraction of PANi could be employed to develop diffraction

based chemical sensor [110]. PPy has been used as sensor material for certain

components in gas mixtures, for both inorganic (NO2, CO2, CO, NH3, H2S) and

organic (acetone, methanol, ethanol) compounds [71, 111].

The widespread interest of electrochemists in these polymers has been spurred by

their applicability in the area of chemically modified electrodes. Coating the

electrodes with electroactive polymers helps in the development of new materials

with very active catalytic properties. In most electrocatalytic systems, the polymer

itself is inert and serves only as a support for the electrocatalytic metal sites. The

electrocatalyst site functions as a mediator, facilitating the transfer of electrons

between the electrode and the substrate.

Electrocatalysis in general is of great economic importance and the aim of these

modified electrodes is to drive electrochemical reactions selectively and/or at modest

potentials, and with better control than could be possible by the direct interaction

between the substrate and the electrode. Fig. 2.8 shows the scheme for a conducting

polypyrrole based sensor in an electrolyte medium of dilute hydrochloric acid

undergoing oxidation and reduction processes.

54

Figure 2.8: Schematic representation of electrochemical oxidation and reduction

of a polypyrrole film.

Catalysis of electrochemical reactions are determined by the availability of reactive

sites at the working electrodes coupled with other hydrodynamic conditions that

promotes attainment of balance between adsorption of reactants and desorption of

products from the electrodes. The electrostatic binding of electroactive ions into

ionomeric polymer films generates nanoscopic electrocatalytic layers. This has a

concomitant effect of shifting the electrode kinetics to a region of lower formal

potential at the operating potential window. Through such shifts in the formal

potential, problems of electroactive interferences are reduced. Many metals such as

tin, bismuth, tungsten, palladium, lead, antimony, selenium and carbonaceous inert

material have been used to modify electrodes for increased reactivity [108].

Optimum preparation conditions for modified electrodes of PANi, PPy and PTh on Pt

have been reported [109]. The immobilization of catalytically active compounds on

55

the conducting polymer film produced by electrochemical polymerization is an

efficient method for production of electrode surface with high stability and catalytic

stability. Processable and nanostructured conducting polymers of PANi, PPy, PTh

stabilized with appropriate surfactants and other hydrogelic polymer composites are

attractive raw materials in building electro catalytic chemical and biological sensors.

This is readily achieved by encapsulating enzymes on the polymeric nanotubes or

films, thus providing ‘wiring’ for electron transfer between the analyte and the

biosensor [111, 113]. The process usually involves any of these; diffusion of substrate

to the film; substrate partition and diffusion into the layer; charge transportation

within the polymer modified electrode and the electrode surface or a mediation

kinetics [114]. Figure 2.9 shows the scheme for a typical mediation process of a redox

couple O/R (O + ne- → R), for the reduction of electroactive analyte. At nanoscopic

level, NCPs offer enough binding sites that are readily immobilized to convey

sufficient signals, while at the same time being thin enough to obtain quick and fast

response. Using cyclic voltammetry such electro catalytic reactions take place at

lower potentials with or without peak currents amplification [115].

e-

Polymer film

R

O O

R Z

Y

Addition of analyte Adsorbed enzyme

Electrode

Figure 2.9: Scheme for the mediation process of an immobilized polymeric film

at an electrode surface for electroactive analyte (Y).

In the review by Wilson et al., an exhaustive report on recent research efforts and

technology on chemical sensors for portable, handheld, field instrument was given

[116]. Generally, large polymeric anions are more firmly fixed than small inorganic

56

anions on the film and consequently increase the rate of electro deposition at the

electrode. X-ray microanalysis has shown that porphyrin doped PPy films were firmly

fixed and were not expelled even during extensive potential cycling [117].

Electrodes formed from PTh polymeric matrix do show higher current densities and

are more stable towards successive potentiodynamic cycles in formic acid oxidation

than their PANi and PPy counterparts [108, 111]. This has been traced to the greater

affinity for electrons by the sulphur atoms of the thiophene rings [118]. The catalytic

effects of the conducting poly (3-methylthiophene)-film electrode on the redox

behaviour of catecol compounds have been reported. It was established that the redox

electron transfer step is taking place at the polymer/solution interface involving the

heteroatom of the thiophene ring at the interface [118].

Applications of NCPs as suitable matrix for the entrapment of biological components

such as enzymes for use as a medium for rapid electron transfer at electrode surfaces

have attracted much interest in electrocatalysis. The advantages derived through this

are enhanced speed, sensitivity and versatility of biosensors in diagnostics to measure

vital analytes. The review of Aduloju and Wallace gave a concise report of earlier

work on this [119].

Mathebe et al. [120] have been reported the fabrication and characterization of PANi-

based amperometric biosensor incorporating Horseradish peroxidase (HRP) for the

detection of hydrogen peroxide (H2O2) in aqueous medium. The catalytic cathodic

reduction of H2O2 was studied amperometrically and also by cyclic voltammetry, both

cases showed a positive correlation between the peak currents and increasing

concentration of H2O2. The sensitivity of the catalytic sensor was found to be

dependent on the concentration of the immobilized enzyme. A range of 0.6 and 0.7 g/l

was recommended as optimum enzyme concentration. Higher concentrations generate

a lower response due to a sterically hindered surface for electron transfer while at

lower concentration a lower sensor response is obtained. Similar improved electro

catalytic response was obtained when functionalized poly (pyrrole/cyclo dextrin)

electrode was used to investigate the detection of some neuro transmitters [121].

57

Catalytic accumulations of analytical signals (by over five orders of magnitude) based

on gold nanoparticle substrates have been used to generate high sensitivity and high

selectivity in nucleic acid determinations [122]. High sensitivity polythiophene based

transducer for the fluorimetric determination of nucleic acid hybridization based on

the electrostatic bonding between positively charged polythiophene and a single

stranded oligonucleotide probe have been reported [123]. The principle involved is

based on monitoring changes in electrical/optical properties of PTh when associated

with the single- or double- stranded nucleic acids.

Recently, Yokoyama et al. [124] reported a digital simulation of the cyclic

voltammetry of reversible electrochemical reactions coupled with enzyme-mediated

reactions. The model is applicable to any substrate and mediator system and the

voltammogram changes with changes in the concentration of the enzyme or the

mediator. As the substrate concentration is depleted in the vicinity of the electrode,

the catalytic current is reduced. Similarly, Temsamani et al. [125] reported the use of

conducting polymer film of polypyrrole doped with sulphonated-β- cyclodextrin

(PPy-SβCD) for the solid phase micro extraction of cationic analytes. In another

application, β-cyclodextrin doped polypyrrole was employed to fabricate an

electrochemical sensor for the selective, simultaneous and quantitative detection of

some polyhydroxyl phenyls and neurotransmitters derived from pyrogallol and

catechol [126].

Generally, the scope of application of nanostructured conducting polymers is grossly

dependent on the intrinsic conductivity, thermal stability and morphology of the

doped polymer. The conductivity of PPy films doped with dopant mixtures of

naphthalene sulphonic acid and HCl was reported to show more thermal stability than

those from single dopants. Similarly, NCPs physical structure is radically different

between different monomer starting groups with the same counter ion, or between the

same monomer groups and with the different counter ions. These physical changes are

useful in chemical sensing as small changes in the polymer synthesis conditions can

lead to differing sensor responses and selectivity [127, 128]. PPy doped polymer with

fibrilar morphology was found to show better conductivity than those with globular

morphology because the former offers a better interaction between polymer chains

[129, 130].

58

Recently, glassy carbon electrode immobilized with conducting nanostructured

polypyrrole doped with naphthalene sulphonic acid is reported to electrocatalyse the

oxidation of phenol at a potential of about 80 mV [56], as against about 850 mV vs.

Ag/AgCl for its normal oxidation on unmodified glassy carbon electrode. In another

report, when exfoliated graphite electrode was used for phenol oxidation, the

irreversible conversion to quinone compounds occurred at 420 mV [131].The

electrocatalysis of conducting polymers could be further enhanced when modified

with dispersed particles of platinum. A thin film of nanoparticles of

polyaniline/platinum composite electrode showed significant electrocatalysis for the

oxidation of hydrazine and the reduction of dichromate [132]. Similar electrocatalysis

is expected for metal nanoparticle/PPy and PTh polymer composites.

A comprehensive review on the applications of electropolymerized conducting

polymers in amperometric biosensors as well as current tendencies and the advances

made in the field was published recently [133]. Special attention was paid to novel

applications, which have opened up new possibilities and lines of research with regard

to NCPs. The review covered applications in areas of imprinted electrosynthesised

film, screen printing technology, immunoassay analysis, immobilization of enzymes

on conducting polymer surfaces, multi-enzyme systems, miniaturization of electrodes

for ‘in vivo applications, liquid chromatography electrochemical detection, organic

phase biosensors and other miscellaneous applications for food and environmental

applications. A lot of analytical and technological development is expected in these

areas in the next few years.

2.9 Future challenges in the scope of conducting polymer applications

The development of novel and improved sensor devices with good electro catalytic

properties is a major challenge for scientists in the next few decades. Improved

processability for existing nanomaterials in appropriate medium, the production of

new materials (synthetic metals) through doping of hitherto insulating polymers and

development of terrific methods that are highly selective must be harnessed. Sensor

materials with good electro catalytic property, reversible and reproducible

electrochemistry must be assembled in portable and cost effective fashion. This

59

should cover all forms of biosensors for health, immunosensors, DNA biosensors,

environmental monitoring biosensors and chemosensors for food and allied industries.

Reproducibility of nanoparticles with desirable morphology and properties for

electrocatalytic application should be of utmost concern to synthetic chemists. It was

reported recently that the process of stirring during the bulk synthesis of polyaniline

leads to the formation of highly dispersible nanofibres [134]. The authors reasoned

that homogenous nucleation of nanoparticles of PANi results in nanofibres while

heterogeneous nucleation leads to granular particulates. They submitted that avoiding

stirring during the oxidation of the monomers could enhance the processability

problem of PANi. In the same manner, the effect of agitation on nanoparticles of

polypyrrole and polythiophene should be more critically investigated.

There is the challenge of developing new electrode systems from the combination of

separate chemical sensors having different transducer technologies to produce new

hybrid electronic nose (E–noses) systems for various quality control applications. The

problem of electrode surface fouling at high temperature by sulphur compounds in

calorimetric sensors should be curtailed. Likewise the humidity levels should be

optimized in polymer-based biosensors for optimal analyte discrimination in sensor

assemblies. There is also the challenge of developing greater photovoltaic

applications for nanostructured conducting polymers. In the near future, more

photovoltaic coatings that gather and emit electricity should be developed to power

and/or protect our houses and vehicles. This will be environmentally friendly and

possibly cheaper than petroleum products. Conductive polymeric coatings that can

fight corrosion will be more stable when applied as car paints. There is equally the

need to rapidly revolutionalise the world with an accelerated commercialization of the

research findings of material science so that the potential benefits therein could be

tapped. In this respect, Analytical Chemist and Engineers have the challenge of

producing microsytems of nanostructured conducting polymers through

simplification, automation and miniaturization of electro analytical processes such

that the analytical problems of selectivity and sensitivity are solved and more so that

the final sensor material is cost effective [135].

60

2.10 Chemical sensors for anthropogenic organic pollutants.

A chemical sensor can be described as a device, which upon exposure to a gaseous or

liquid chemical compound or mixture of chemical compounds alters one or more of

its physical properties (e.g. mass, electrical conductivity, or capacitance) in a way that

can be measured and quantified directly or indirectly. Sensors can be classified

according to their operating principle, each class having a different sensitivity and

selectivity. The sensor signal may be due to a change in the resistance

(conductometry), a change in potential difference across the polymer interface

(potentiometry), or to generation of oxidation or reduction of the analyte

(voltammetric/amperometric).

The working principle of a typical electrochemical sensor is illustrated in Figure 2.10.

Different sensors could be used singly or in an array to generate patterned recognition

and identification of various analytes.

Pt/PPYNQSA

Analyte Polymer Transducer Amplifier Monitor Output layer

Figure 2.10: Scheme for the working principle of an electrochemical sensor e.g.

electronic nose, (EN).

0102030405060708090

1. Kw 2. Kw 3. Kw 4. Kw

61

The analyte molecules interact with the polymer interface in a lock and key

mechanism to generate an electrical signal through the chosen transducer. The

generated signal is amplified and monitored with an artificial intelligence such as

computer hardware/software to produce measurable analytical signal. Furthermore, it

is possible to change the properties of the sensor by changing parameters (e.g. sensing

material or temperature) during manufacturing or operation of the sensor in order to

improve selectivity and sensitivity. For example, by an inclusion of different metal

ions into the polymer, the sensor can be adjusted for various chemical species. It is

well known that electron conducting polymers can be used as chemical sensors by

measuring the conductivity changes as a function of secondary doping or undoping of

detected species [136]. In order to improve the characteristics of PPy based

composites as chemical sensors; the sensitivity could be tuned by modifying the

nature of the interacting polymer site with the analyte of interest.

Hwang et al. had proposed a microscopic gas-sensing model indicating that the

sensitivity of the ethanol-based PPy-PEO composite film sensor depends on the site

number of a monolayer, the thickness of the sensing film, the adsorption equilibrium

constant as well as the change of site resistance [137, 138]. It was also found out that

the ethanol sensing abilities of the PPy-PEO sensor are better at lower polymerisation

charge [139]. The design of gas sensors was based on determining the sorption

properties on the polymers surface. Chehimi et al. [140] had reported that conducting

polymer surface is amphoteric with polar molecules (Lewis acids and bases) being

adsorbed to a greater degree than non-polar molecules. The polymers surface

morphology thus plays important role in surface thermodynamics of the polymer.

Conducting polypyrrole based electronic sensor arrays have been developed for toxic

and non-toxic substances, such as ammonia, nitrogen, nitrogen oxides, carbon

monoxide, sulphur dioxide, hydrogen sulphide, methane, oxygen, hydrogen, alcohols,

phenols, benzene and water vapour in materials ranging from water and beverages to

waste waters and sewage effluents [139].

The conductivity of an intrinsically conducting polymer like pyrrole is simply

measured at a constant current or voltage over a resistor. Sorption of gases or liquid

(containing the analyte) into the polymer matrix and the interaction with the matrix

62

causes a change in the conductivity of the polymer. The sensor response is not

necessarily a linear relationship between the analyte concentration and conductivity.

Responses are typically measured as the relative differential resistance (R2-R1)/R1,

where R1 is the baseline resistance in clean air or electrolyte and R2 the resistance in

analyte atmosphere. In the study by Lin et al. [136], on the responses of PPyCl sensor

to BTEX compounds, ο-xylene caused increase in conductivity being an electron-

donating compound. The resistance change (∆R) increases linearly with ο-xylene

concentration from 20 to 60 ppm in N2. The sensitivity was 0.8 mΩ/ppm. The

sensitivities obtained for BTEX compounds were in the following order ο-xylene › m-

xylene being ≈ p-xylene › ethylbenzene › toluene › benzene. This shows decrease from

the polar to non-polar compounds.

Hydrogen bonding and dipole-dipole interactions are also reported to play important

roles in sensing process [141]. The infrared spectra of a PPy film after exposing to

acetone indicated the formation of hydrogen bonds (H-bonds) between C=O groups of

acetone molecules and N−H groups of pyrrole units [141]. Also, the conductivities of

conducting polymers are directly determined by the doping levels and the properties

of counter ions. For example, the conductivity of ClO4－ doped PPy is higher than that

of p-toluenesulphonate (TsO－) doped PPy [141].

Change in the morphology of conducting polymer’s sensing layer can strongly

influence the performances of chemresistors and diodes. This is attributable to the

results of changing the ratio of surface area to volume (rA/V) [143]. A film with higher

rA/V makes analyte molecules diffuse and interact with the sensing layer more easily,

which lead to a higher sensitivity and shorter response time. The exposure of

conducting polymer samples to the vapour of volatile substances can have their

conductivity altered in a reproducible manner. The combined pattern of variation of

the individual conductivities of slightly different polymeric substances in an assembly

could be used to give a specific ‘signature’ of the volatile compound. This is the

operating principle of the conducting polymer based “electronic noses” currently in

developmental stages in different laboratories [20, 139, 144, 145]. A comprehensive

review of the achievements from the various research efforts from 1994 to 2005 on

PPy-based E-Noses for environmental and industrial analysis is contained in [139].

63

Nanofibers (wires, tubes etc) have huge rA/V values, so they are perfect candidates for

preparing polypyrrole sensors with high sensitivities and fast responses. In this study,

careful control of the polypyrrole morphology have been done using surfactant

dopants of naphthalene sulphonic acid and 1,2-naphthaquinone-4-sulphonic acid; and

transition metal oxides of tungsten oxide and zirconium oxide to tune the interaction

of polypyrrole to some anthropogenic organic pollutants.

2.10.1 Surfactant modified polypyrrole chemical sensors

Surfactants are water-soluble agents made up of hydrophilic heads and hydrophobic

tails. They are widely used in soaps, detergents, pharmaceuticals and biotechnology

and can also be used in the modification of conducting polymer. Improved

processability of polypyrrole is usually obtainable by modification of the polymer’s

structure through the use of substituted monomer, variation in the type of oxidant and

also that of the doping material. However, the mostly studied approach to structural

variation of a conducting polymer like PPy is to modulate the properties through the

incorporation of sulphonic acid group (surfactants) which helps to improve the

polymers solubility in water, and allows the possibility of self doping. Selection of a

particular surfactant for a particular application is dependent on the critical micelle

concentration (CMC), i.e. the concentration at which it starts to form micelles. CMC

decreases as the carbon chain length and molecular weight increases, and the addition

of surfactants into aqueous solutions increase the conductance of the solution [146].

Different approaches have been explored such as use of aliphatic or aromatic organic

acids. In the study by Yasuo Kudoh [147], highly conducting and environmentally

stable PPy was prepared by chemical polymerization using aqueous solution

containing Fe2(SO4) as oxidant and an anionic surfactant of either sodium

dodeclybenzenesulfonate (NaDBS), sodium alkylnaphthalenesulfonate (NaANS) or

sodium alkylsulfonate (NaAS) . In the study, aromatic sulfonate was found to have a

stronger tendency to increase the moisture and thermal stabilities of the polymer than

alkylsulfonate. The improved thermal and environmental properties of the aromatic

sulphonated polypyrroles were ascribed to the large-sized surfactant anions that were

effectively incorporated into PPy as dopant. Evidence of the incorporation of the

monovalent sulfonate anions as dopant into PPy was monitored by the increase in the

64

S/N ratio. The doping ratio of each dopant was calculated by the following

simultaneous equations:

x + y = S/N ratio

2x + y = total doping ratio (constant)

where x and y are the doping ratios of sulfate (oxidant) and sulfonate (dopant),

respectively. A yield in excess of 100 % was reported when excess DBS was

adsorbed on the PPy surface.

Furthermore, the addition of the surfactant into the pyrrole-monomer solution was

reported to have accelerated the polymerization reaction [147]. It was reported that

the yield of the modified PPy is proportional to the surfactant concentration until near

the maximal dopable concentration after which a lower constant conductivity value

was maintained. The lower conductivity at the higher surfactant concentration region

was ascribed to the presence of surfactant anion adsorbed on the PPy surface [147].

The effectiveness of the surfactant modified PPy was dependent on the electron

withdrawing nature of the -SO3H- dopant moiety, the choice of dopants and steric

effect from the dopant being used, the choice of oxidant and its relative ratio to the

monomer mass concentration.

In order to improve selectivity of polypyrrole-based sensors towards acetone and

toluene which are flammable components of lacquer over the non-flamable acetic acid

and water, Ruangchuay et al. used blends of α-naphthalene sulfonate (PPy/α-NS-)

with common insulating polymers to create differences in surface composition and

wetting ability [148]. The PPy/α-NS- was more sensitive to acetone and toluene but

less sensitive towards water, relative to pure PPy/α-NS-. The sensitivity towards acetic

acid was not significantly different from that of pure PPy/α-NS-. Similarly, Jun et al.

used different pre-treatment procedure to use dodecylbenzenesulphonic acid (DBSA)

doped polypyrrole to generate different response times of the polymer to different

alcohol gases and volatile organic compounds such as acetone and chloroform [149].

Using an array of different sensors based on polypyrrole, eight different sufonated

polypyrroles were identified by Barisci et al. to detect and quantify benzene, toluene,

ethylbenzene, xylene i.e. (BTEX) compounds [150]. The uses of PPy gas sensors ENs

65

for various industrial and environmental materials have been critically reviewed

[139].

Generally the sensing principle of chemical sensors is that it transforms the

concentrations of analytes to other detectable physical signals, such as currents,

absorbance, mass or acoustic variables. After exposing to the vapour of an analyte, the

active sensing material of the sensor will interact with the analyte, which causes the

observed change in the physical property of the sensing material. The interactions

between the analytes and sensing materials are multiform, according to different

analytes and different active materials [141].

2.10.2 Transition metal oxide modified polypyrrole sensors

Another approach in improving the processability and application of polypyrrole is to

prepare the polymer composite using metal oxide template. The ability of transition

metals to exhibit multiple oxidation states makes metal-oxide doped conducting

polymers as suitable intermediate material for the catalytic exchange of electrons in

many heterogeneous electrochemical systems. This also is aimed at synergistically

improving selectivity and stability. Metal oxide doped polypyrrole is the hybrid

polymer obtained from the chemical or electrochemical coupling of electrically

conducting metal oxides with polypyrrole. These polymers find potential applications

in electrochemical systems such as sensors, batteries and fuel cells [151].

The basic components of a metal oxide semiconductor (MOS) are shown in Fig. 2.11.

Figure 2.11: Scheme for the metal oxide semiconductor sensor.

66

Rocco et al. fabricated an electrochromic device combining PPy and WO3 in 1996

[152]. The device consisted of tin doped indium oxide (ITO) coated with

PPy/dodecylsulfate, an ITO electrode coated with WO3 and a liquid junction. The

light filtering capacity and stability of the solid-state device was dependent on the

thickness of the PPy film. The chromatic contrast was stable after 15,000 double

potential chromatographic steps [153].

Many Metal Oxide Semiconductor sensors (MOS) from materials such as TiO2, WO3,

In2O3 and other oxides have been used in the assembly of the different chemical

sensor systems, metal field effect transistors (MOFSET) and as gas sensing elements

in E-noses [151]. While conducting polymer based chemical sensors could be used at

ambient temperature, MOS sensors are used at elevated temperatures. Doping the

metal oxide with noble catalytic metals can be used to modify the selectivity of the

MOS devices through the changing of working temperature of the sensing element

(250-400 ºC), or by modifying the grain size [153]. Figure 2.12 shows conducting

polymer sensor used to measure changes in conductance/resistance at different

operating conditions. It does not require the use of a reference electrode.

Figure 2.12: Scheme of a conductometric based sensor using a conducting

polymer material.

2.10.3 Polycyclic aromatic hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants

that originate from diverse anthropogenic sources. They are usually common in the

vicinity of gas manufacturing plants as a result of incomplete combustion processes of

organic carbon-based material [155]. PAHs are hydrophobic compounds with low

67

water solubility. They are of increasing interest because of their toxic, mutagenic and

carcinogenic properties [156].

PAHs do accumulate in the environment because they are thermodynamically stable

compounds, due to their large negative resonance energies; they have low aqueous

solubilities, and they adsorb to soil particles. Because of these reasons, PAHs do not

readily undergo natural environmental processes including biodegradation. Therefore,

PAHs persist for long periods under many natural conditions in the contaminated

environment [157]. The major natural source of PAHs in the environment is

combustion of biomass. Other significant sources are petroleum and fossil fuels, coal

and lignite, and coal tar residue from coal gasification processes. The major industrial

sites polluted with PAHs are coal gasification and wood-preserving plants. Some

PAHs such as naphthalene are also obtained from the concentration of the high

boiling residual oil (and asphalt) derived from crude petroleum refinery processing.

These PAHs are mostly used as intermediaries in pharmaceuticals, agricultural

products, photographic products, thermosetting plastics, lubricating materials, and

other chemical industries [158].

PAHs exist in various combinations that manifest various functions. They are made

up of two or more fused benzene rings and some "pentacyclic moieties" in linear,

angular, and/or cluster arrangements. The simplest example is naphthalene having two

benzene rings side by side while others could vary from 2, 3 and more fused aromatic

rings. The United States (US) Environmental Protecting Agency (EPA) proposed a

list of 16 PAHs in 1984 as carcinogenic or mutagenic compounds [159], which was

adopted by the US government in 1988. Some of these PAHs are shown in Figure

2.13. Further concern on PAHs necessitated the European Union to adopt a legislation

in 2005 [160] that provided a list of 15 PAHs (8 in common with US EPA and 7 new

compounds) which were of major concern for human health due to their toxic

properties (Table 2.1). The general characteristics of PAH includes high melting- and

boiling points (they are solid), low vapour pressure, and very low water solubility,

decreasing with increasing molecular weight. However, resistance to oxidation,

reduction, and vaporization increases with increasing molecular weight. Vapour

pressure tends to decrease with increasing molecular weight. PAHs are highly

lipophilic and readily soluble in organic solvents. The lower molecular weight PAHs

68

of 2 or 3 ring groups such as naphthalenes, fluorenes, phenanthrenes, and anthracenes

have toxicity that tends to decrease with increasing molecular weight [159].

Naphthalene Fluoranthene

Anthracene Pyrene

Phenanthrene

Figure 2.13: Structural representations of some PAHs included in the USEPA

list [158].

69

Table 2.1 Chemical structure of the 15 European Union (EU) priority PAHs

[161]

70

The main environmental impact of PAHs relates to their health effects, focusing on

their carcinogenic properties. The semi-volatile property of PAHs makes them highly

mobile throughout the environment via deposition and re-volatilisation between air,

soil and water bodies. Most of the PAHs released in the environment end up being

deposited in the oceans and/or subject to long range transport making them a

widespread environmental problem. Although PAHs may undergo chemical

oxidation, photolysis and volatilization, microbial degradation is the major process

affecting PAH persistence in nature [157].

It is therefore desirable in order to put in place adequate clean up programmes for the

prevention and remediation of PAHs pollution in the environment to avoid public

health hazards. Consequently, companies prone to generate these materials for

installation of abatement equipment at factory locations and also at old sites for the

purposes of monitoring control and remediation of PAHs are committing a lot of

investment. There are limits for work place exposure for chemicals that contain PAHs

such as coal tar and mineral oil. The Occupational Safety and Health Administration

(OSHA) has set a limit of 0.2 milligrams of PAHs per cubic meter of air (0.2 mg/m3).

The OSHA Permissible Exposure Limit (PEL) for mineral oil mist that contains PAHs

is 5 mg/m3 averaged over an 8-hour exposure period. Likewise, the National Institute

for Occupational Safety and Health (NIOSH) recommends that the average workplace

air levels for coal tar products should not exceed 0.1 mg/m3 for a 10-hour workday,

within a 40-hour workweek. There are other limits for workplace exposure for things

that contain PAHs, such as coal, coal tar, and mineral oil [158].

Therefore, there is an ever-increasing demand for the determination of trace amounts

of these substances at plant sites, water bodies and air. So far, mostly

chromatographic methods such as GC-MS or HPLC with fluorimetric detection are

used for these purposes [161]. However, these methods are characterized by high

investment and running costs. Modern electroanalytical methods are required for the

determination of these dangerous chemical carcinogens. The main advantage

derivable from this electroanalytical method is a much lower investment and running

costs. A study of voltammetric determination of trace amounts of carcinogenic 1-

nitropyrene and 1-aminopyrene using a glassy carbon paste electrode by Barek et al.

was described recently [162]. A fiber coating from polyaniline (PANI) was

71

electrochemically prepared and employed for solid phase microextraction (SPME) of

some polycyclic aromatic hydrocarbons (PAHs) from water samples. The study

revealed that this polymer is a suitable SPME fiber coating for extracting the selected

PAHs [156]. The development of more electrochemical devices for these

anthropogenic pollutants still remains a big challenge to scientists.

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Chapter Three

Analytical Techniques and Experimental Procedures

This chapter gives a synopsis on the different analytical techniques employed,

detailed research methodology and general experimental procedures for the chemical

and electrochemical preparation, characterization and application of the modified

polypyrrole electroactive materials.

3.1 Analytical techniques

The analytical techniques used in this study are electrochemical techniques namely,

cyclic voltammetry (CV), Oyster-young square wave voltammetry (OSWV),

differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy

(EIS); spectroscopic techniques namely Ultra Violet-Visible spectroscopy (UV-Vis),

Fourier transform infra red spectroscopy (FTIR), substractively normalised in-situ

Fourier transform infra red spectroscopy (SNIFTIRS); and morphological technique

which was mainly scanning electron microscopy (SEM).

3.1.1 Electrochemical techniques

Two broad electrochemical techniques were used in these studies namely:

voltammetric and impedimetric techniques. The merits derivable from the techniques

used are discussed below.

3.1.1.1 Cyclic Voltammetry

Cyclic voltammetry (CV) is the most widely used electrochemical technique for the

characterisation of redox systems. It provides information about the number of

oxidation states, as well as qualitative information about the stability of these

oxidation states and quantitative information about the rates, mechanisms and the

electron transfer kinetics [1]. Modern electroanalytical measurements are normally

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performed with software driven potentiostats, two examples of which are shown in

Fig. 3.1.

(a) AUTOLAB Voltammetric instrument

(b) CV 50 Voltammetric instrument.

Figure 3.1: Typical electroanalytical instrument used for cyclic voltammetry [2].

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In a CV experiment, as in other controlled potential experiments, a potential is applied

to the system, and the faradaic current response, resulting from a redox reaction is

measured. The potential is cycled between a pre-determined potential window from

an initial potential, Ei, to a final (switching) potential, Ef, at a constant scan rate (ca.

from 1 mV/s to a few thousand mV/s). By varying the potential limits, the reactivity

of the electrochemical system is probed over a large range of potentials in a single

sweep. Also by varying the sweep rate, the kinetics of the reactions and/ or mass

transfer processes is probed [3]. The potential scan may be terminated at the end of

the first cycle or continued for any number of cycles leading to a multisegment

voltammogram. The cycling could be done oxidatively (increasing potential) or

reductively (decreasing potential) [4]. The forward half of the CV is identical to a

linear-sweep voltammogram. When the potential is scanned in a positive direction,

the electroactive species at the surface of the electrode is oxidised and if it is scanned

in the negative direction, they are reduced. The oxidation generates a current response

as a result of the depletion of species at the electrode surface. The plot of the applied

potential vs. the resulting currents provides characteristic peak potentials for both

oxidation (Ep,a) and reduction (Ep,c), and their corresponding peak currents (Ip,a and

Ip,c) respectively. The peak width at half the peak current (Ep1/2) is also sometimes

measured. This characteristic feature of the voltammogram provides information

about the redox (formal) potential of a system denoted as E°′ [4].

The chemical processes involved in redox cycling are complicated, thus, the CV could

be reversible (limited by diffusion); irreversible (completely limited by kinetics) i.e.

the reverse reaction is so slow at electrode potentials larger than the redox potential;

or quasi-reversible (partially limited by kinetics) [3]. A redox couple in which both

species rapidly exchange electrons with the working electrode is termed an

electrochemically reversible couple. Figure 3.2 shows typical cyclic voltammogram

for the electrochemical oxidation and reduction process. The formal reduction

potential (E°′) for a reversible or quasi-reversible couple is at the mid-point of Ep,a and

Ep,c on the voltammogram and it is commonly determined from equation 3.1 [1, 4].

E°′ = (Ep,a + Ep,c) / 2 equation 3.1

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Figure 3.2: Typical cyclic voltammogram for the electrochemical oxidation and

reduction process.

The number of electron transferred (n) can be determined from the separation between

Ep,a and Ep,c (∆Ep) using equation 3.2 [1, 4].

∆Ep = (Ep,a - Ep,c) = 0.059 / n equation 3.2

Thus, for a one electron process, a ∆Ep of 0.059 V will be expected.

The number of electrons transferred (n) can also be determined via Tafel plots. This

plot relates the generated current to the overpotential, η, in an exponential manner

similar to the Arrhenius and Eyring equations. At a fixed temperature, T, the Tafel

equation is:

I = a + b exp η equation 3.3 [1].

A plot of log10 I (as ‘y’) against overpotential, η, (as ‘x’), called Tafel plot is usually

linear over a narrow range of potentials and only valid for high overpotential. As a

general rule it is stated that the Tafel equation is valid for η > 118/n mV, where n is

the number of electrons exchanged [5].

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http://en.wikipedia.org/wiki/Image:Cyclovoltammogram.jpg

For low overpotentials, the Buttler Volmer approach is applied whereby the net

current (Inet) from the oxidation and reduction is given by equation 3.4 [1, 4].

Inet = Io [exp (α n F η/RT) - exp (1-α n F η/RT)] equation 3.4

Where Io is the exchange current representing the rate constant of electron transfer at

zero overpotential, α is the transfer coefficient, F is the Faradays constant, R is the

molar gas constant and T is the operating temperature in Kelvin. The actual value of

this current (Inet) is affected by many additional factors, most importantly the

concentration of the redox species, the size, shape, and material of the electrode, the

solution resistance, the cell volume, and the number of electrons transferred.

Cyclic voltammetry can also provide information about the rate of electron transfer

between the electrode and the analyte, and the stability of the analyte in the

electrolyzed oxidation states (e.g., do they undergo any chemical reactions). For a thin

layer of adsorbed electro active material at the electrode surface undergoing Nerstian

reaction, the plot of peak current values versus scan rates for both cathodic and anodic

peaks respectively show linear dependence. This is in accordance with Brown-Anson

model, equation3.5, from which the surface concentration of the various redox states

could be estimated [4, 6, 7].

Ip = n2 F2Γ*Aν / 4RT equation 3.5

Ip = Peak current for either the oxidation or reduction peak being considered

n = Number of electrons transferred

F = Faraday constant (96584 C mol-1)

Γ* = Surface concentration of the electroactive film bound to the working

electrode

A = Surface area

ν = Scan rate (Vs-1)

R = Gas constant (8.314Jmol-1K-1)

95

T = Temperature of the system (K).

By varying the scan rates (Vs-1) of the process, the diffusion coefficient, De, (which

tells how rapidly the electroactive species is diffusing through the solution to and

from the surface of the working electrode) can be calculated using the Randles-Sevčik

equation [4, 7],

Ip = - 0.4463 n F (nF/RT) 1/2 Γ* De1/2 ν1/2 A equation 3.6

This reduces to equation 3.7 at 25 °C for A in cm2, De in cm2/s, Γ* in mol/L, and ν in

V/s, Ip in amperes

Ip = 2.686 x 105 n3/2 A Γ*De1/2 ν1/2

equation 3.7

Accordingly, Ip increases with ν1/2 and is directly proportional to concentration. The

relationship to concentration is of particular interest in analytical applications and in

studies of electrode mechanisms. A plot of Ip versus ν1/2 should give a straight line

from which De can be evaluated. For a reversible system, the values of Ipa and Ipc

should be identical for a simple reversible (fast) couple. However, the ratio of the

peak currents can be significantly influenced by chemical reactions coupled to the

electrode process.

The standard rate constant (kº) for electron transfer within the polymer chain could be

estimated using Nicholson treatment for a quasi-reversible electrochemical system

The net shape of the voltammetric curves have been shown to depend on a kinetic

parameter φ which is given by the expression:

φ = [(DO/DR) α/2 kº / (π.n. De.F. ν/RT) 1/2] equation 3.8

where for simplicity the diffusion coefficient (De), for the oxidized (O) and reduced

(R) forms of the solution phase probe redox couple are equal, i.e. DO = DR = De; This

simplifies to:

96

kº = φ (α. n. F. ν. π. De / RT) ½ equation 3.9 [7].

For a typical reversible system, α = 0.5. As the systems turns from reversible to

irreversible, there is a transition in the magnitude of α and ΔEp. For 0.3 < α < 0.7, the

ΔEp values are nearly independent of α and depend only on φ. Thus estimates of kº

could be made using the tabulated values of φ for different values of scan rates [4].

A numerical analysis of the diffusion boundary value problem for cyclic voltammetry

has established a quantitative analysis of the relationship between the kinetic

parameter φ and the voltammetric peak separation ΔEp. This has been plotted as a

working curve and result summarised for reference purposes in different literatures [4,

7, 8]. While a ΔEp value of 61 mV gives a φ value of 20 at 25oC, the φ values at 63

mV and 212 mV are 7 and 0.10 respectively [4]

3.1.1.2 Oyster-young square wave voltammetry (OSWV)

The theory of SWV was invented by Ramaley and Krause in 1969 [9] but was only

extensively developed by Osteryoung and co-workers in 1986 [10]. The technique

involves the application of square wave modulation to a constant or nearly constant dc

potential, and the current generated is sampled at the end of successive half cycles of

the square wave. Three currents are generated, vis: forward current from the forward

pulse (if), the reverse current from the reverse pulse (ir) and that for the net current (Id)

vs. the potential on the corresponding staircase tread. The net current serves better

analytical usage than the forward and reverse currents because it increases the

discrimination against the charging current, since any residual charging current is

subtracted out. Figure 3.3 shows the OSWV containing the forward, reverse and

reverse currents. The net current is larger than that for the forward and reverse current

since it is the difference between them [11]. The peak height is directly proportional

to the concentration of the electroactive species and direct detection limits as low as 1

x 10–8 M is possible as against 5 x 10–8 M differential pulse voltammetry and 1 x 10–5

M in cyclic voltammetry [1].

SWV has many advantages over other differential techniques such as much faster

scan times, excellent sensitivity, the rejection of background current, high signal to

97

noise ratio and applicability to a wider range of electrode materials and systems. [12].

Oyster-young square wave voltammetry can play very important role in the

characterization of electroactive species with poor, overlapping or ill-formed redox

signals in cyclic voltammetry by producing individual, sharp peaks [11].

Figure 3.3: Typical OSWV for PPyNSA film containing the forward, reverse and

reverse currents.

3.1.1.3 Differential pulse voltammetry

Differential pulse voltammetry is one of the voltammetric pulse techniques just as

Oyster-young square wave voltammetry (OSWV) and normal pulse voltammetry

(NPV) which are largely applicable for trace voltammetric analysis at a working

electrode [1]. In DPV, a succession of differential pulses (dE/dt) of fixed, but small

amplitude (10-100 mV) is superimposed on the working electrode. Two currents are

generated, one taken immediately before applying the potential pulse and the second

is taken late in the pulse. The differential of the two currents is displayed on the

potential-current curve. At potentials, well positive of the redox potential, there is no

faradaic response to the pulse, so the differential current is close to zero. At potentials

98

around the redox potential, the differential current reaches a maximum and decreases

towards zero, as the current becomes diffusion-controlled. The current response is

therefore a symmetrical curve. Figure 3.4 shows a typical differential pulse

voltammogramm for a film of polypyrrole doped with 1, 2-naphthaquinone-4-

sulphonate (PPyNQS) on a platinum working electrode in an undegassed 0.1 M

LiClO4 at a scan rate of 5 mV/s and 25 mV amplitude. The peaks at the positive

potentials are characteristic for polypyrrole while the peaks at the negative potentials

are characteristic for dissolved oxygen in the electrolyte solution.

.

Figure 3.4: Typical potential-current curve for the anodic and cathodic

differential pulse voltammetric scan of polypyrrole in 0.1 M LiClO4 at a scan

rate of 5 mV/s and 25 mV amplitude.

Differntial pulse voltammetry is widely used for the identification of any electroactive

specie at the electrode surface, which will be largely used in characterising the

conducting polymers used in this work. Individual redox specie generates a

corresponding symmetrical peak. The potential of the peak can help identify the

cation in solution in a similar manner to normal polarography. The peak area is

proportional to concentration. The peak height could be used as approximation for the

99

estimation of an analyte’s concentration using the Osteryoung-Parry equation

(equation 3.10) [1].

ΔIp = (n2F2A/4RT). (D/πt) 1/2. Canalyte. ΔE equation 3.10

Where all terms have their usual meaning and t is the time between pulses. The

magnitude of ΔE and the rooted term in brackets, (D/πt) ½, implies that the separation

of potential and diffusion of analyte to the electrode plays important role in

determining the value of ΔIp which increases accuracy of the technique. DPV is very

versatile in its scope of application just as the OSWV. Its advantages over NPV are

twofold: (i) many analyte could be sampled with a single voltammogram since the

analytical peaks for each analyte are well resolved, and (ii) by working with a

differential current the sensitivity of the technique is improved. While the lower

detection limit for DPV is 10-8 to 5 x 10-8 M, that for NPV is only 10-7 to 10-8 M [1].

3.1.1.4 Electrochemical Impedance Spectroscopy

In cyclic voltammetry and other dynamic electroanalysis, an applied potential is either

constant (potentiostatic) or changing (potentiodynamic) when ramped at a constant

rate of V = dE/dt. However, in impedance, a small perturbing potential is applied

across a cell or sample that changes in a cyclic sinusoidal manner and generates a

current resulting from the overpotential (η) caused by the small displacement of the

potential from the equilibrium value. Over a time scale, the averaged over potential is

zero. Because the potential is only perturbing, it has the advantage of minimising the

concentration change after the experiment. The induced current alternates because the

voltage changes in a cyclic manner, and hence the term alternating current (AC). The

term impedance is therefore a measure of the ability of a circuit to resist the flow of

an alternating current (AC). It is synonymous to resistance (R) used in direct current

(DC), which is defined by Ohm’s law (equation 3.11) as the ratio between voltage (E)

and current (I) [1, 4].

R = E/I equation 3.11

100

During a controlled-potential electrochemical impedance spectroscopy (EIS)

experiment, the electrochemical cell is held at equilibrium at a fixed DC potential, and

a small amplitude (5–10 mV) AC wave form is superimposed on the DC potential to

generate a response from the equilibrium position. The response to the applied

perturbation, which is generally sinusoidal, can differ in phase and amplitude from the

applied signal. This response is measured in terms of the AC impedance or the

complex impedance, Z, of the system, which permits analysis of electrode process in

relation to diffusion, kinetics, double layer, coupled homogeneous reactions, etc [13].

The ratio of the applied voltage (E) over measured current (I) is the impedance of the

system (Z = E/I). Since an AC potential is applied to the cell, there will probably be a

phase shift by an angle (φ) between the applied AC potential waveform and the AC

current response. Therefore, the impedance can be represented using a vector diagram

(figure 3.5) displaying the in-phase and out-of-phase impedances, the total

impedance, and the phase angle (φ).

Figure 3.5: Sinusoidal current response to potential perturbation as a function of

time.

Depending on the AC perturbation, the potential and current functions at a particular

frequency could be represented by the equations 3.12 and 3.13:

101

E(t) = Eo sin (ωt) equation 3.12

and,

I(t) = Io sin (ωt + φ) equation 3.13.

Where E(t) is the potential at t, Eo is the amplitude of the signal, and ω is the radial

frequency (radians/second) defined as 2 π f with f being the frequency in hertz, I(t)

and Io represent the response current signal and amplitude respectively. Using ohms

law, an analogous expression for the impedance could be derived by substituting

equations 3.12 and 3.13 into equation 3.11 as shown below:

Z = Et/It = Eo sin (ωt)/ Io sin (ωt +φ) equation 3.14

Z = Zo * sin (ωt)/ sin (ωt +φ) equation 3.15

Since complex number terminology is involved when analyzing impedance spectra,

the in-phase and out-of-phase impedances are often referred to as real and imaginary

impedances. The complex impedance (Z) is made up of a resistive or real part Z′,

attributable to resistors (in phase with the applied voltage), and a reactive or

imaginary part Z′′, attributable to the contributions of capacitors (out of phase with

the applied voltage by π/2) and /or inductors (out of phase with the applied voltage by

-π/2). This is related to the resistance (R), reactance (X) and capacitance (C) by the

equation:

Z = R – j X equation 3.15

where X = 1/ωC and ω = 2 π f. R is the resistance measured in Ohms (Ω), X the

reactance, C the capacitance measured in Farads (F), ω the applied angular frequency

measured in rad/s and f is the frequency measured in Hertz (Hz) [4].

Notational representation of this in terms of Z′ and Z" is given by:

102

Z = Z′ - j Z′′ where j = √-1 equation 3.16

Because Z is defined by the complex term, j, which determines the contribution of Z′′

to Z, the term complex impedance is often used. For a pure resistor that is not having

any capacitance, its resistance when determined with a continuous current (DC) is R

because its impedance is frequency independent, Z = Z′ = R.

The experimental data collated from an impedance experiment is often presented as

Nyquist plot of Z′ (usually positive x-axis corresponds to the real impedance), versus

Z′′ (usually, the positive y-axis correspond to -Z"), over a wide frequency range

(normally 100 kHz to 0.1 Hz). Another way of presenting impedance data is a Bode

plot in which the logarithm of the absolute value of Z′ and the phase (φ) are plotted

against the logarithm of the frequency (f) [14]. This could be plotted together or

separately. Nyquist plots is more commonly displayed for historical reasons, the data

is however often poorly resolved (particularly at high frequencies), and the explicit

frequency dependence is not displayed. In contrast, the Bode plot directly displays the

frequency dependence; in addition, the data is well resolved at all frequencies, since a

logarithmic frequency scale is used.

When the frequency of the AC waveform is varied over a wide range of frequency (ca

about 10-4 and > 106 Hz), the impedance obtained for the system is a function of the

operating frequency. Spectrums of the resulting impedance at different frequencies do

reveal the different electrochemical kinetics involved in the system. While dipolar

properties are manifest at the high frequency regions, bulk and surface properties will

be evident at intermediate and low frequencies respectively [14]. The total impedance

of a system is determined by the impedances of the various components of the

electrochemical cell; for example, electron transfer kinetics, diffusion, passivating

layers, etc. The relative contribution of the various components typically varies with

frequency; for example, electron transfer kinetics may dominate at high frequencies,

whereas diffusion may dominate at lower frequencies.

Measuring impedance over a wide frequency range allows processes with different

time scales to be detected within the same experiment. The Nyquist plot obtained for

103

a particular system could therefore be used for both qualitative and quantitative

analysis. Some important electrochemical parameters about a NCP can be

simultaneously gotten from a single experiment at a known level of charge. This

includes charge transfer at the metal (electrode) /film interface, the rate of charge

transfer in the film, ohmic resistance, double layer capacitance and redox capacitance

of the film and even the potential dependence of the diffusion coefficient for ionic and

electronic charge carriers [4, 14]. To achieve this, the practical constraint is choosing

the appropriate equivalent circuit and an accurate fitting procedure to obtain the

different variables.

While qualitative information could be obtained by mere inspection of the EIS

spectrum, quantitative information requires proper use of appropriate software (Z-

View or Z-Plot) to model the electrical components of the system to equivalent

electrical circuits. One common method is based on the principle of equivalent circuit;

that is, the various cell components can be modeled using electronic components (e.g.,

a resistor for electron transfer kinetics and solution resistance, and a capacitor for the

interfacial (double layer) capacitance), and an electronic circuit can be built that gives

the same impedance spectrum as the electrochemical cell. This approach requires that

each equivalent circuit element corresponds to a component of the electrochemical

cell and a good match must exist between the experimental impedance spectrum and

the model impedance spectrum over the entire frequency range, otherwise the

equivalent circuit has no meaning.

Impedance data is commonly analysed by fitting it to an equivalent circuit model. The

frequently used circuit, called the Randles equivalent circuit is composed of different

elements such as resistors, capacitors, and inductors joined in series and/or in parallel.

Figure 3.6a shows the Nyquist plot for the real impedance (x-axis) and imaginary

impedance (y-axis) with low frequency data being on the right side of the plot and

higher frequencies are on the left. A typical Nyquist plot for the impeadnace data

obtained on platinum electrode in ferricyanide solution is shown in Fig 3.6b. The

semicircular shape is characteristic of a single "time constant". A representative plot

of frequency as x-axis versus logarithm of real impedance as y1-axis, and phase angle

as y2-axis called (Bode plot) is presented in Figure 3.7 for impedance spectra obtained

104

on the system presented in Fig. 3.6b showing the variation of phase angle and

impedance with frequency.

(a)

(b)

Figure 3.6: Nyquist plot with impedance vector shown as “a” [14] and typical

Nyquist plot of Ferricyanide solution on platinum electrode.

105

10m 1 100 10K

100

300

1K

3K

10K

30K

100K

-90

-75

-60

-45

-30

-15

0

frequency / Hz

|impedance| / Ω phase / o

Figure 3.7: Typical Bode plot of Ferricyanide solution on platinum electrode

showing variation of impedance and phase angle with changes in frequency.

Electrochemical Impedance plots often contain several time constants, but often only

a portion of one or more of their semicircles is seen. The shape varies depending on

the equivalent electrical circuits for the system. Fig. 3.8 is a typical equivalent circuit

of a capacitor and a resistor in parallel. This is discussed further underneath.

C1

R1

Figure 3.8: Equivalent circuit of a capacitor and a resistor in parallel (one time

constant).

3.1.1.4.1 Electrical circuit elements

Any electrochemical cell can be represented in terms of an equivalent electrical circuit

that comprises a combination of resistances and capacitances. There could also be

106

contribution of inductances at very high frequencies. Contributions to the resistance of

a cell are the solution resistance (Rs), the charge transfer resistance (Rct), Warburg

impedance (Zw) while contribution to the capacitance could be as a capacitor (C) and

constant phase element (CPE) [1, 14]. These elements are described briefly

underneath.

Solution resistance (Rs): The solution resistance is the resistance between the

working electrode and the reference electrode. This is indicated as a small offset on

the real impedance axis. It is measured at high frequency intercept near the origin of

the Nyquist plot. The resistance of an ionic solution depends on the ionic

concentration and type of ions the electrolyte is made up, temperature and the

geometry of the area in which current is carried. In a bounded area with area A and

length l carrying a uniform current the resistance is defined as:

equation 3.17

The conductivity of the solution, k, is more commonly used in solution resistance

calculations. Its relationship with solution resistance is:

equation 3.18

The units for k are siemens per meter (S/m). The siemens is the reciprocal of the ohm,

(1 S = 1/ohm).

Charge transfer resistance (Rct): This is the resistance associated with the charge

transfer mechanisms for electrode reactions. It is the resistance to electron transfer at

the electrode interface. It is deduced from the kinetically controlled electrochemical

reaction at low over-potentials. From the Buttler-Volmer equation (equation 3.4), the

net current (Inet) from the oxidation and reduction reactions is:

Inet = Io [exp (α n F η /RT) - exp (1-α n F η /RT)] equation 3.4 [1, 4].

When (n F η /RT) is well below unity, the Buttler-Volmer equation could be

linearised to obtain

107

I = Io n F η / RT equation 3.19 [4]

And also in terms of over-potential (η) and concentration terms for oxidised, reduced

and equilibrium states for a one electron redox process,

η = RT/F [CO (0, t)/CO* – CR

(0, t)/CR* + i /io] equation 3.20 [4].

Hence, when the over-potential, η, is very small and the electrochemical system is at

equilibrium, the expression for the charge transfer resistance for an n-electron process

changes into:

equation 3.21 [3, 4].

From this equation the exchange current density (Io) can be calculated when Rct is

known. The charge transfer resistance is estimated from the diameter of the

semicircular region on the real impedance axis of the Nyquist plot. When the

chemical system is kinetically sluggish, the Rct will be very large and may display a

limited frequency region where mass transfer is a significant factor. However, if the

system is so kinetically facile, and the mass transfer always plays a role, the

semicircular region is not well formed [4].

Warburg Impedance (ZW): This is the resistance associated with the diffusion of

ions across the electrode/electrolyte interface. This impedance is associated with the

difficulty of mass transport of electroactive species [13]. Layers of ions at the

electrode interface behave like an RC element (i.e. a resistor and a capacitor in

parallel) and this produces infinite sum of RC elements called the Warburg

impedance. It is characterised as a linear portion at an angle of 45° and slope of unity

on the Nyquist plot and a slope of -0.5 on the Bode plot [1].

Capacitor (C): The capacitance (C) is the ability of an electrochemical system to

store or retain charge. An electrical double layer exists on the interface between an

electrode and its surrounding electrolyte. This double layer is formed as ions from the

solution "stick on" the electrode surface. The potential at the terminals of this double

108

layer (capacitor) is proportional to its charge. The impedance of a capacitor is given

by the equation

Z (C) = Z′′ = 1/jωC equation 3.22

Constant phase element (CPE): A constant phase element is a non-intuitive circuit

element that was invented while looking at the response of real-world systems. Often,

a CPE is used in a model in place of a capacitor due to deviation of capacitance

parameters from expected values. In some systems the Nyquist plot was expected to

be a semicircle with the center on the x-axis. However, the observed plot may be an

arc of a circle with the center being some distance below the x-axis. These depressed

semicircles have been linked to a number of phenomena, depending on the nature of

the system being investigated. This behaviour was traced to the non-homogeneity of

the system or that there is some distribution (dispersion) of the value of some physical

property of the system. The impedance of a CPE is represented by equation

CPE = equation 3.23 [15]

= 1 / (C jω) α equation 3.24 [13]

This is similar to that of a capacitor except that the constant A = 1/C (the inverse of

the capacitance) and the exponent α = 1 for a true capacitor. For a constant phase

element, the exponent α is less than one. When α = 0.5, a 45° line is produced on the

complex plane graph and could be used for an infinite length of Warburg element.

During circuit fitting, the CPE is defined by two values, i.e., the capacitance, C, and

the CPE exponent, α, which has a value between 0.5 and 1 for a non-ideal capacitor.

If n equals 1, the equation is identical to that of a capacitor and smaller values can be

related to surface roughness and in-homogeneities, which lead to frequency

dispersion.

3.1.1.4.2 Impedance modeling using equivalent electrical circuit

Jiri Janata [16] has identified three major points to be considered in equivalent circuit

analysis for chemiresistors as follows:

109

(a) Description of individual physical processes by realistic electrochemical

circuit elements;

(b) Arrangement of this elements in a rationally constructed equivalent circuit;

(c) Optimisation of the values of the elements in the equivalent circuit such that

the overall transfer function (response) matches the response of the real cell.

The external current flowing in an electrochemical cell (voltaic cell) made up of two

metal electrodes A and B (figure 3.9) serving as contact to an electrolyte solution of

their salt is given by the equation

Icell (ω) = Ecell (ω)/Zcell (ω) equation 3.25 [16]

Figure 3.9: General representation of an electrochemical cell.

The overall cell impedance is sum of the impedances in the path of the cell current,

Zcell (ω) = ZA + ZB + ZElectrolyte equation 3.26 [16]

Considering the commonest combination of a capacitor and a resistor arranged in

parallel, the flow of current will be divided into two branches, namely: IR and IC in

line with Kirchoff’s law.

. ICell = IR + IC equation 3.27

110

Impedance spectrums are often modeled using an electrical circuit which produces a

similar spectrum as that from the experimental data. The electrical components

(resistors, capacitors, inductors, etc) and some 'components' that have no electrical

analogue (constant phase elements, Warburg impedances, etc) are then matched to

physical characteristics of the measured cell [13, 17]. Some of the following

predefined elements are used in impedance fitting during the course of this work

R Resistor

C Capacitor

L Inductor

CPE Constant Phase Element

Ws Warburg - Short Circuit Terminus

Wo Warburg - Open Circuit Terminus

The impedance of a resistor is constant at all frequencies. A pure resistor is usually

represented as a single-point on the real impedance (Z’) axis of the Nyquist plot.

However, the impedance of a capacitor decreases as the frequency is increased. The

C values are infinitive at frequency of zero but having zero value at infinite

frequency. Capacitors have only the imaginary component of impedance, the real

impedance is zero. The impedance of an inductor increases as frequency increases.

Notational and mathematical descriptions of these behaviours are presented

underneath. For all equations: j = square root (-1), and ω = angular frequency of the

AC signal.

1. R – Resistor (Z = R)

R

Z′ = R Z’’ = 0

2. C – Capacitor (Z (C) = Z′′ = 1/jωC)

C

111

Z′ = 0 Z′′ = 1/jωC

3. L = Inductor (Z = jωL)

L

Z′ = 0, Z′′ = ωL

4. CPE = Constant Phase Element

CPE

Z = 1 / (C jω) α

For kinetically favoured reactions, Rct → 0 and Zw predominates and for difficult

reactions Rct → ∞ and Rct predominates. When other steps are involved in the

electrode process, homogeneous or heterogeneous, more complicated circuits are

involved. A simple circuit commonly used is the Randles circuit (Figure 3.10).

Rs Cd

Z

Figure 3.10: Randles circuit for a simple electrochemical cell. Rs is the solution

resistance, Z is the impedance of the electrode process and Cd is the double layer

capacity.

Various combinations of appropriate circuits in series or/and parallel will be explored

in modeling the reactivities at wide range of frequencies for the polypyrrole materials

and sensors using Z-View software.

The impedance and capacitance generated by the perturbation of a stationary working

electrode with small AC voltage (5 – 10 mV) amplitude provides bulk and interfacial

electrical information on the material connected to the transducer. When this

perturbation is done over a large range of frequencies (ca. 106 to 10-4 Hz), a spectrum

112

for the various electro-kinetics involved in the system will be reflected at different

frequency regions [4]. While dipolar properties of solid materials will be revealed at

high frequencies, bulk and surface properties becomes evident at medium and low

frequencies [14]. In the production of an impedance sensor for selective hydrocarbon

gas sensing, Hagen et al used a normalized impedance parameter Sn, to follow the

interfacial reaction at a novel zeolite based electrode [18]. The methodology involved

measuring deviations of the magnitude of Z at a time t (Zt) from a value without

hydrocarbon at time t = 0, (Z0) divided by the hydrocarbon concentration at time t = t,

(Zt). The sensor effect in this work is based on similar normalized real impedance

(Sn), and/or that of the normalized real capacitance (Kn) as shown in equation 3.28

and 3.29.

Sn = |Zt| - |Z(t = 0)| equation 3.28 [18]

|Z(t = 0)|

Kn = |Ct| - |C(t = 0)| equation 3.29 [18]

|C(t = 0)|

3.1.2 Spectroscopic techniques (UV-Vis, FTIR, SNIFTIRS)

Two broad spectroscopic techniques were employed in this study, namely: Ultra

Violet-Visible spectroscopy (UV-Vis) and Fourier transform infra red spectroscopy

FTIRS. In-situ FTIR studies were also carried out using substractively normalised in-

situ Fourier transform infra red spectroscopy (SNIFTIRS).

3.1.2.1 Ultra Violet-Visible spectroscopy (UV-Vis)

UV – Vis absorption spectra were recorded at room temperature on a GBC UV/ Vis

920 spectrophotometer (GBC Scientific Instruments, Australia) between 200 and 900

nm using a 1-cm path length quartz cuvette and 99.6% dimethyl sulfoxide (DMSO) or

dimethyl formamide (DMF) as reference solvent. UV-Vis measurements were made

with the filtrate obtained from dispersions of the polymer materials in appropriate

113

solvent. Spectra obtained in each case were processed and investigated for

characteristic absorptions that could be linked to the electrochemistry of the materials.

The band gap peculiarities of the materials were explored using the wavelengths of

maximum absorption.

3.1.2.2 Fourier Transform Infra Red (FTIR)

The Fourier Transform Infra Red (FTIR) spectra were recorded on a Perkins Elmer

FT-IR Spectrometer, Paragon 1000PC. In each case, less than 0.0010 g of each

polymer was ground in a medium of 0.4 g of dried KBr salt, and placed in the pallet to

obtain a fairly transparent pellet. The spectra were recorded in the wavenumber region

of 400 to 4000 cm-1. The characteristic set of absorption bands in the spectrum were

used to identify various functional groups predominating in the various polymeric

states.

3.1.2.3 Subtractively Normalized Interfacial Fourier Transform Infrared

Spectroscopy (SNIFTIRS) [19, 20, 21]

Subtractively normalized interfacial Fourier transform infrared spectroscopy

(SNIFTIRS) measurements were performed in the staircase mode using a fully

evacuated FTIR spectrometer (Bruker IFS113v) fitted with a mercury cadmium

telluride (MCT) photo detector and Ge/KBr beam splitter. Thin film of polypyrrole

was subjected to SNIFTIRS analysis using a three-electrode cell similar to the cyclic

voltammetry set-up. The reflectance data on the film at the working electrode was

collected at different applied potential using a manually controlled potentiostat. A

calomel reference electrode was used and a platinum ring electrode fitted on the

inside of the custom made FTIR electrochemical cell, served as counter electrode.

During the in situ measurements of FTIR spectra the surface of the working electrode

was positioned very close to the CaF2 window and parallel to it in order to limit the

influence of solvent on the spectra.

Spectra were obtained at potentials from 0 mV to 600 mV at 100 mV intervals.

Spectra were also recorded at selective potentials in the reverse direction as a check

on the reversibility of the modified polymer. Spectra were obtained by Fourier

transformation after averaging 200 interferograms acquired at each potential, using p-

114

polarised radiation. Infrared spectra have been normalized with respect to the

reference spectrum collected at 0 mV and are displayed as ΔR/R difference spectra

(the ratio of the signals obtained at potentials Ei and Eref and that obtained at the

reference potential Eref) in units of reflectance, since no logarithm was applied [20].

Subtractively normalized in-situ FTIR spectra obtained in this way therefore, contain

only information of the molecular changes occurring from modification of the

oxidation state of the polymer. Negative bands were interpreted as indicative of

absorption by species generated as the potential is changed to Ei, while upward peaks

were interpreted as the disappearance of initial species [21].

3.1.3 Morphological technique (SEM)

Scanning electron microscopy was used for morphological investigation in this study.

Polypyrrole samples were examined under a scanning electron microscope (Hitachi

X650 Micro-analyzer) with interchangeable accelerating voltages of 10kV to 30kV

for optimum sensitivity. In the SEM experiment, about 0.01g of the polypyrrole

sample was spiked onto a carbon coated sample holder charged with gold to improve

surface electrical conductivity. The charged sample is subsequently transferred into

the micro-analyzer where it is transversed by electron beam. The signals produced are

collected by an appropriate detector, amplified and displayed on the cathode ray tube

at different magnifications. The magnification of the image is the relationship

between the length of the scan line on the specimen and that on the cathode ray tube.

Energy Dispersive X-ray (EDX) spectra and the elemental analysis for carbon,

oxygen and sulphur were captured on the polymers micrograph by stigmation of the

relevant area on the micrograph where the nanoparticles were assayed.

3.2 Chemical synthesis procedures and characterization of polypyrrole

nanomaterials.

The general experimental procedure used for the chemical synthesis of polypyrrole

was as described in literature [22-26]. This involved usage of different dopants,

temperature, concentrations of dopant and oxidant in the mother liquor, etc.

Polypyrrole was prepared from distilled water, hydrochloric acid, β-naphthalene

sulphonic acid, sodium salt of 1, 2-napthaquinone-4-sulphonic acid, tungsten oxide

and zirconium oxide. The characterisation techniques are as described previously.

115

3.2.1 Chemicals

All chemicals used in this study were purchased from Sigma – Aldrich (Pty) Ltd.,

South Africa. The pyrrole (98%) was re-distilled at reduced pressure and saturated

with argon atmosphere and stored in 1 mL ampoules in the dark at 4 oC. β-

naphthalene sulphonic acid (70%), sodium salt of 1, 2-napthaquinone-4-sulphonic

acid (BDH laboratory reagent), ammonium peroxodisulfate (98%), acetone (99.8%),

hydrochloric acid (32%), and methanol (99.8%) were used without further treatment.

Deionised-distilled water used was prepared with Milli-Q water purification apparatus

(Millipore). Solvents used for spectroscopic studies are 99.6% dimethyl sulfoxide

(DMSO) or N, N-Dimethylformamide (DMF)-A.C.S spectrophotometric grade.

3.2.2 Chemical synthesis of β-naphthalene sulphonic acid doped polypyrrole

(PPyNSA); polypyrrole from distilled water (PPyDW); and polypyrrole from

HCl (PPyHCl)

A series of polypyrroles was synthesized by varying the reaction conditions reported

for the chemically synthesized polypyrrole doped with dodecyl-benzene sulphonic

acid (DBSA) [22, 23]. β-Naphthalene sulphonic acid was used as surfactant and

dopant [24, 25, 26]. In a typical procedure for the preparation of PPyNSA from a

dopant to monomer mole concentration ratio (d/m) of 0.8 and an oxidant (APS) to

monomer concentration ratio (o/m) of 0.2, 1.059 mL (0.015 mol) of pyrrole was

dissolved and stirred in 20 mL aqueous solutions of 3.5696 g (0.012 mol) of NSA at

60 oC while stirring for 15 min. The reaction solution was cooled and 10 mL aqueous

solution of oxidant (APS) containing 0.7607 g (0.0030 moles), was added and stirred

for 24 hr at 0 oC. The beakers were rinsed with distilled water to make up the total

volume of liquor to 50 mL. Similarly various polypyrroles were synthesized using

d/m mole concentration ratios of 0.5 and 1 and o/m mole concentration ratios of 0.5

and 1 at 0 oC and room temperature of 25 oC. Control experiments were performed in

NSA-free aqueous solutions (HCl or water) of the monomer to produce NSA-free

polypyrroles tagged PPyDW or PPyHCl. Polymerization process was terminated in

each case by the addition of excess methanol and the liquor allowed to age. The

resulting polypyrrole precipitate was vacuum filtered and washed sequentially with

excess distilled water, methanol and acetone until a colourless filtrate was obtained.

116

The precipitates obtained were dried in a vacuum oven at 25 oC for 12 hours. The

apparent yield (g/mL) was determined by calculating the weight (g) of dry powder to

initial volume (mL) of monomer.

The table showing relative amounts of materials used for the separate preparations is

presented underneath (Table 3.1). The results from these syntheses were used to

investigate the following:

Effect of pH on polypyrrole synthesis

Effect of temperature on polypyrrole synthesis

Effect of type, presence and concentration of dopant on polypyrrole synthesis

Effect of concentration of oxidant on polypyrrole synthesis

Morphological examination of polypyrrole from different synthetic conditions

Spectroscopic examination of polypyrrole from different synthetic conditions

Electrochemistry and application of chemically synthesised polypyrrole

117

Table 3.1: Table of relative amounts of materials used for chemical synthesis of

PPyNSA, PPyDW and PPyHCl.

Code Pyrrole

used

NSA

/dopant

used

APS used Ratios pH color change

NSA-

1

1.059 mL

(1.5 x 10-3

mol)

3.4620 g

(1.5 x 10-3

mol)

0.7607 g

(3.0 x 10-3

mol)

d/m (1.00)

o/m (0.20)

1.4 Milky white to

black

NSA-

2

1.059 mL

(1.5 x 10-3

mol)

3.5696 g

(1.2 x 10-3

mol)

0.7607 g

(3.0 x 10-3

mol)

d/m (0.80)

o/m (0.20)

1.50 Milky white to

black

NSA-

3

1.059 mL

(1.5 x 10-3

mol)

2.2310 g

(7.5 x 10-3

mol)

0.7607 g

(3.0 x 10-3

mol)

d/m (0.50)

o/m (0.20)

1.70 Milky white to

black

DW 1.059 mL

(1.5 x 10-3

mol)

(Synthesis

in distilled

water)

0.7607 g

(3.0 x 10-3

mol)

d/m (0.00)

o/m (0.20)

1.80 White to black

HCl-

1

1.059 mL

(1.5 x 10-3

mol)

(Synthesis

in 0.1 M

HCl)

0.7607 g

(3.0 x 10-3

mol)

d/m

(<0.10)

o/m (0.20)

1.69 White to black

HCl-

2

0.529 mL

(7.5 x 10-3

mol)

6.0 mL of

1M HCl

(6.0 x 10-3

mol)

0.3457 g

(1.5 x 10-3

mol)

d/m (0.80)

o/m (0.20

0.22 White to black

HCl-

3

0.529 mL

(7.5 x 10-3

mol)

3.8 mL of

1M HCl

(1.5 x 10-3

mol)

0.3457 g

(1.5 x 10-3

mol)

d/m (0.50)

o/m (0.20

0.44 White to black

118

3.2.3 Chemical synthesis of 1, 2-naphthaquinone-4-sulfonate doped polypyrrole

(PPyNQS)

Using the same experimental procedure as in 3.2.2 above, 1,2-naphthaquinone-4-

sulfonate doped polypyrrole (PPyNQS) were prepared using the sodium salt of 1, 2-

napthaquinone-4-sulphonic acid as the surfactant dopant and pyrrole monomer in

aqueous solution of hydrochloric acid (acting as the supporting electrolyte). It has

been observed that when a chemical material which on its own could orchestrate the

oxidation of monomer is used as dopant in very small amounts, the subsequent

polymerisation would be on the nanofibre seeds of the dopant and not necessarily on

the surfactant acting as a polymerisation template [27]. Usage of NQS has intrinsic

property of being able to pre-oxidize pyrrole to form nanofibres which generate the

bulk polymer with further oxidation with the APS, or with the application of potential.

Following the over-oxidation observed in the use of a d/m ratio of 0.8, and o/m ratio

of 0.2, lower doping ratios were explored to arrive at an optimised d/m ratio of 0.05

and o/m ratios of 0.2 and 1. This study investigated preparations using d/m mole

concentration ratios of 0.8 and 0.05; and o/m mole concentration ratios of 1.0 and 0.2

at 0 oC based on the optimised conditions established from 3.2.2. Table 3.2 presents

the summary of relative amounts of materials used for the preparations.

In a typical preparation of PPyNQS using d/m mole concentration ratios of 0.8 and

o/m mole concentration ratios of 1.0 at 0 oC, 0.529 mL (0.0075 mol) of pyrrole was

dissolved and stirred in 20 mL aqueous solutions 1.540 g (0.0060 mol) of NQS at 60 oC while stirring for 15 min. The reaction solution was cooled and 10 mL aqueous

solution of oxidant (APS) containing 1.7287 g (0.075 mol), was added and stirred for

24 hr at 0 oC. Similarly, using d/m mole concentration ratios of 0.8 and o/m mole

concentration ratios of 0.2 at 0 oC, 0.529 mL (0.0075 mol) of pyrrole was dissolved

and stirred in 20 mL aqueous solutions 1.540 g (0.0060 mol) of NQS at 60 oC while

stirring for 15 min. After cooling, 10 mL aqueous solution of oxidant (APS)

containing 0.3457 g (0.0015 mol), was added and stirred for 24 hr at 0 oC. Synthesis

using d/m mole concentration ratios of 0.05 and o/m mole concentration ratios of 0.2

at 0 oC involved use of 0.529 mL (0.0075 mol) of pyrrole and 0.0982 g (3.776 x 10-4

mol) of NQS as described earlier. Control experiments were performed in NQS-free

aqueous solutions of the monomer to produce NQS-free polypyrroles tagged PPyDW.

119

Polymerization process was terminated in each case by the addition of excess

methanol and the liquor allowed to age. The resulting polypyrrole precipitate was

vacuum filtered and washed sequentially with excess distilled water, methanol and

acetone until a colourless filtrate was obtained. The precipitates obtained were dried

in a vacuum oven at 25 oC for 12 hours. The apparent yield (g/mL) was determined by

calculating the weight (g) of dry powder to initial volume (mL) of monomer.


PPyNQS.

Code Pyrrole

used

NQS/dopant

used

APS used Ratios pH color change

NQS

-1

0.529 mL

(7.5 x 10-3

mol)

1.540 g

(6.0 x 10-3

mol)

0.3457 g

(1.5 x 10-3

mol)

d/m (0.80)

o/m (0.20)

1.41 Dark brown to

black

NQS

-2

0.529 mL

(7.5 x 10-3

mol)

0.0982 g

(3.776 x

10-4 mol)

0.3484 g

(1.5 x

10-3 mol)

d/m (0.05)

o/m (0.20)

1.40 Yellowish

brown solution

to black

NQS

-3

0.529 mL

(7.5 x 10-3

mol)

0.0982 g

(3.776 x

10-4 mol)

1.745 g

(7.5 x

10-3 mol)

d/m (0.05)

o/m (1.00)

1.43 Yellowish

brown solution

to black

3.2.4. Chemical synthesis of metal oxide doped polypyrrole (PPyMO) using

Tungsten oxide (WO3) and Zirconium oxide (ZrO2)

Similar procedure as used for PPyNSA and PPyNQS was employed in preparing

polypyrrole composites using two separate metal oxides, namely tungsten (VI) oxide

(WO3) and zirconium (IV) oxide (ZrO2). Table 3.3 and 3.4 presents the reacting

materials used for the synthesis of PPyWO3 and PPyZrO2 respectively.

120


PPyWO3.

Code Pyrrole

used

WO3 used APS used Ratios pH Colour

change

WO3-

A

0.529 mL

(7.5 x 10-3

mol)

1.391 g

(6.0 x 10-3

mol)

0.3484 g

(1.5 x 10-3

mol)

d/m (0.80)

o/m (0.20)

1.68 Light green to

black after

oxidation

WO3-

B

0.529 mL

(7.5 x 10-3

mol)

1.391 g

(6.0 x 10-3

mol)

1.745 g

(7.5 x 10-3

mol)

d/m (0.80)

o/m (1.00)

1.36 Light green to

black after

oxidation


PPyZrO2.

Code Pyrrole

used

ZrO2 used APS used Ratios pH Colour

change

ZrO2-

A

0.529 mL

(7.5 x 10-3

mol)

0.740 g

(6.0 x 10-3

mol)

0.3484 g

(1.5 x 10-3

mol)

d/m (0.80)

o/m (0.20)

1.77 White milky

solution to

black

ZrO2-

B

0.529 mL

(7.5 x 10-3

mol)

0.740 g

(6.0 x 10-3

mol)

1.745 g

(7.5 x 10-3

mol)

d/m (0.80)

o/m (1.00)

1.37 White milky

solution to

black

3.3 Electrochemical synthesis and characterization procedures

3.3.1 Apparatus

All voltammetric experiments (CV, OSWV, and DPV studies) were carried out with a

BAS 50W electrochemical workstation (Bioanalytical Systems, Lafayate, IN, USA) at

121

room temperature. Electrochemical impedance spectroscopy (EIS) measurements

were performed with a PGZ402 Voltalab Analyzer (Radiometer Analytical S.A,

France). A conventional three-electrode cell was used. The electrodes were a 0.0201

cm2 platinum disc or 0.0707 cm2 glassy carbon disc as working electrode (WE), a

platinum wire auxiliary electrode (AE) and a Ag/AgCl (3 M NaCl type) reference

electrode (RE). The WE was cleaned by polishing on slurries of 1.0 μm, 0.3 μm and

0.05 μm alumina powders (Bueller, IL, USA) placed on individual alumina pads, and

rinsed with deionised water obtained by passing distilled water through a Milli-Q

water purification apparatus (Millipore). The surface of the disc WE was polished by

using circular motions on the pad containing 1.0 μm followed by 0.3 μm slurry and

then 0.05 μm slurry. Intermittently between usages, the electrodes were treated in hot

concentrated H2SO4 and 30% H2O2 and washed with distilled deionised water. The

counter electrode (AE) was cleaned between each experiment by heating in a Bunsen

flame, washed and rinsed with copious amount of deionised water. The reference

electrode was rinsed in deionised water between each experiment.

3.3.2 Electrochemical synthesis procedures of modified polypyrrole

While bulk quantities of intrinsically conducting polypyrrole are prepared from

chemical synthesis, thin films of the polymer is preferably electrosynthesised on

suitable working electrodes by careful control of the nature of electrolyte, dopants,

anodes and electrochemical window. Careful control of film thickness and extent of

oxidation is very crucial during processing to avoid over-oxidation of polypyrrole and

subsequent loss of electroactivity. Potentiodynamic and potentiostatic polymerisation

methods were used. However, the potentiodynamic approach was preferred as it gave

more uniform and reproducible results during the course of the research.

Potentiostatic approach was explored during the optimisation of processing conditions

for PPyNSA.

3.3.3 Electrolyte and potential window for polypyrrole synthesis

An initial screening for an appropriate choice of electrolyte for the electrosynthesis

and characterisation of polypyrrole involved use of different concentrations of

hydrochloric acid and sulphuric acid. Films of polypyrrole were prepared from 0.1 M

122

solutions of freshly distilled pyrrole in an ionic solution of 0.1 M HCl with or without

other dopants such as surfactants metal oxides. The choice of 0.1 M HCl was based

on the clear and unambiguous potential window between -400/700 mV which allowed

for characterisation of peaks resulting from polypyrrole within the same potential

window. Use of sulphuric acid (H2SO4) as electrolyte was marred with multiple peaks

resulting from the dual ionisation of the dibasic acid at low scan rates. The choice of a

an electrolyte concentration of 0.1 M HCl was based on the finding that higher

concentration of 0.5 M and 1M HCl causes undesirable and accelerated oxidation of

the pyrrole monomer before application of potential. Besides, ion transportation that

is required for effective polymerisation takes place better in more dilute solutions

according to Debye Huckel theory. It was the satisfactory performance of the 0.1M

HCl as electrolyte/supporting electrolyte that warranted its choice as the electrolyte

medium in this work. In some application stage use of 0.05 M HCl or 0.01 M HCl

was used as reaction medium for sensor reaction.

Potentiodynamic polymerisation of polypyrrole using a potential limit of 500 mV or

less does not generate any polymer. A minimum potential of 600 mV must be applied

to initiate the polymerisation process. Care was taken to avoid over-oxidation of the

polymer by growing the film at the lowest possible upper potential limit between 600

mV to a maximum of 800 mV. At about 850 mV, the irreversible oxidation of

polypyrrole set in. This was in agreement with literature [28]. Further care to avoid

over-oxidation of the polymer was to purge the electrolyte properly by de-gassing

with a gentle flow of argon gas for 15 min prior to usage and keeping the argon

atmosphere on the electrolyte during polymerisation and characterisation processes.

The films were grown potentiodynamically with a scan rate of 50 mV/s. In most cases

a lower and higher scan rates from this do not generate desirable quality of film. The

electrosynthesised films were dried in air for about 2 min prior to characterisation in

fresh hydrochloric acid solution.

3.3.4 Polypyrrole electrosynthesis from aqueous solution of HCl and product

characterization.

The polypyrrole films were prepared by potentiodynamic cycling from 0.1M pyrrole

in 0.1M HCl which had been properly ultrasonicated. Depending on the electrode

123

material, appropriate potential window were chosen for polymerization and

characterization. All the electrochemical characterization (CV, OSWV, DPV, EIS)

were done in an electrolyte solution of 0.1M HCl.

3.3.5 Polypyrrole electrosynthesis from aqueous solution of β-naphthalene

sulphonic acid and product characterization.

The polypyrrole films modified with β-naphthalene sulphonic acid were prepared by

potential cycling from a solution containing the 106-μL pyrrole-monomer (0.0015

mol) and 0.0893 g of β-naphthalene sulphonic acid -dopant (0.0003 mol) respectively

in 15 mL of 0.1 M HCl or 0.05 M HCl. This d/m concentration ratio of 0.2 gave a fair

trend of electropolymerisation of conductive polymer after several attempts to

electropolymerise using higher ratios as used for chemical synthesis failed. The

polymerization and characterization of PPyNSA were carried out separately on glassy

carbon electrode (GCE) and platinum (PtE) disc electrodes at a scan rate of 50 mV/s

using an optimised potential window of -600/+800 mV in 0.05 M HCl. While limited

interrogation could be achieved using voltammetric techniques, more intuitive

findings were obtained using EIS.

All the electrochemical characterizations (CV, OSWV, DPV, EIS) were done in an

electrolyte solution of 0.1M HCl.

3.3.6 Polypyrrole electrosynthesis from aqueous solution of 1, 2-

naphthaquinone-4-sulfonate and product characterization (PPyNQS)

A thin film of conducting polypyrrole was grown under potentiodynamic conditions

from a solution containing the 350-μL pyrrole monomer (0.0050 mol) and 0.130 g of

the sodium salt of 1, 2-napthaquinone–4-sulphonic acid-dopant (0.0005 mol)

respectively in 50 mL of 0.05 M HCl. This solution is made up of a dopant to

monomer concentration ratio of 0.1. Higher d/m ratios caused over-oxidation of

pyrrole. The solution from which the film was generated was then replaced with a

fresh electrolyte solution of 0.05 M HCl, in which all cyclic voltammetry experiments

were then performed. The potential window used for the polymerization and

characterization studies was -400 mV to 700 mV vs. Ag/AgCl. OSWV were

124

performed at 5 Hz at 25 mV square wave amplitude, and a potential step of 4 mV

from an initial potential, Ei, of -400 mV to a final potential, Ef, of 700 mV. DPV

experiments were performed at scan rates of 5, 10 and 20 mV at pulse amplitude of 25

mV. Similarly EIS interrogation of the electrochemical behaviour of the

electrosynthesised film was done at different stepping potentials.

3.3.7 Polypyrrole electrosynthesis from aqueous solution of metal oxide of

tungsten oxide and zirconium oxide

The exploration of the electrosynthesis of polypyrrole films modified with tungsten

oxide or zirconium oxide by potential cycling using platinum and glassy carbon

electrodes was inconclusive. A solution containing 0.529 mL (0.0075 mol) of pyrrole

and 1.391 g (0.0060 mol) of tungsten oxide in 50 mL of 0.1 M HCl was used for

PPyWO3. Similarly the preparation of the PPyZrO2 was explored using a solution

containing 0.529 mL (0.0075 mol) of pyrrole and 0.740 g (0.0060 mol) of zirconium

oxide in 50 mL of 0.1 M HCl. The optimisation of the polymerisation conditions for

the attainment of conductive polymer on the electrode surface was inconclusive.

3.3.8 Electrochemical characterization of chemically synthesised polypyrrole

Thin film of the chemically synthesized polypyrrole were generated from the aqueous

solution of the material via adsorbtion at the glassy carbon electrode (GCE) being

used as the working electrode. A 1- mL cell containing de-aerated saturated paste of

0.025 g of the black polypyrrole powder in 0.5 mL 0.1M HCl was cycled 20 times at

50mV/s from -500 to 1000 mV. The GCE modified with polypyrrole was

characterized by multiscan rate CV, OSWV, DPV and EIS in de-aerated 0.1M HCl.

OSWV was performed at 50 mV square wave amplitude, and 2 mV potential step

from an initial potential, Ei, of -400 mV to a final potential, Ef, of 900 mV. The

forward, reverse and net square wave responses were plotted for frequencies of 2, 3, 4

and 5 Hz. DPV experiments were performed at scan rates of 5 and 10 mVs-1 within

the same potential window. EIS measurements were performed using perturbation

voltage amplitude of 10 mV at different fixed potentials from -600 to 1000 mV in 100

mV or 200 mV intervals during sequential frequency scan from 105 to 10-1 Hz at room

temperature of 25 oC.

125

3.4 Sensor development

The test application using some of the prepared polymers as sensor materials for some

common pollutants in waste waters are presented underneath. The test applications

were based on amperometric and impedimetric transduction techniques.

3.4.1 Chemicals

The chemicals used as analytes in the test application of the sensor materials are

phenol (99%), benzidine (>98.0%) and naphthalene (99%) which were supplied by

Sigma – Aldrich (Pty) Ltd., South Africa.

3.4.2 Phenol sensing with GCE/PPYNSA (Amperometry)

Different concentrations of phenol were analysed in acidic solution containing phenol

using a thin film of β-naphthalene sulphonic acid doped polypyrrole (PPyNSA)

adsorbed on a glassy carbon electrode. The GCE/PPYNSA working electrode used for

phenol sensing was prepared as described in 3.3.8. The electrochemical cell was set

up by placing 1 mL 0.05 HCl (pH 1.2) in a 5 mL cell. The working, reference and

counter (Pt wire) electrode were placed in the solution and connected to the BAS 50

Potentiostat. The solution was degassed for 15 min with argon and an argon head-

space maintained at very low gas flow rate. Square wave voltammetry of the

GCE/PPYNSA working electrode was then measured at 5 Hz using 50 mV square

wave amplitude, and an initial potential, Ei, of -400 mV to a final potential, Ef, of 900

mV. Aliquots of 2.0 mM phenol (0.0471g in 250 mL) were then added to the cell

solution which was degassed (15 min) after each phenol addition, and SWV responses

recorded as described before. Amperometric curve for the GCE/PPYNSA sensor to

phenol addition was plotted from the square wave responses at 70 mV.

3.4. 3 Benzidine sensing with Pt/PPyNQS (Impedimetry)

Stock solution of 0.001 M of benzidine was prepared by dissolving 0.0184 g of

benzidine in 100 cm3 of distilled water (under continuous stirring for 3 hrs). From this

126

stock solution, aliquots (in μL) were drawn and added to the electrolyte in the cell and

used for sensor development. Proper interaction of the analyte with the polymer

surface was achieved by a gentle flow of argon being used for purging.

Impedance measurements

Different concentrations of benzidine in acidic aqueous solution were analysed using

a thin film of 1, 2-naphthaquinone-4-sulfonate doped polypyrrole (PPyNQS)

electrosynthesised on a platinum working electrode. The Pt/PPyNQS working

electrode used for benzidine sensing was prepared as described in 3.3.6. A PGZ402

Voltalab Analyzer (Radiometer Analytical S.A, France) was used for all the EIS

measurements using the same three-electrode cell arrangement and electrolyte as for

cyclic voltammetry. An AC amplitude of 5 mV was imposed on the potential applied

on the working electrode at a frequency range from 100 KHz down to 0.01 Hz at a

sampling rate of 10 points per frequency decade. A voltammetric cell containing 2

cm3 of 0.05 M HCl electrolyte was used for the application tests. EIS data was first

taken using 2 mL of 0.05 M HCl free of analyte and thereafter dosed with progressive

aliquots (in μL) of benzidine. All electrochemical measurements were taken under an

inert atmosphere of argon.

EIS data was modeled by fitting the impedance data using a simple RsR1C1-R2C2

circuit. These elements represents the solution resistance (Rs) between working and

reference electrode, a R1CPE1 parallel combination, where CPE is a constant phase

element, to model movement by electron hopping through the polymer film along the

polymer backbone, and a second R2C2 component in series representing the electrode

/solution interface. The CPE was modeled as a non-ideal capacitor. Thus two

simultaneous kinetics are involved, one involving the bulk polymer material which is

observable at high frequency and the other taking place within the electrolyte/polymer

interface at the low frequency end. The capacitance and impedance values at 10 KHz

for the bulk polymer data and the values obtained at 10 mHz as were used to represent

the electrolyte/polymer interface. The sensor effect was based on the normalized real

impedance (Sn), and/or that of the normalized real capacitance (Kn) using equations

3.28 and 3.29

127

3.4.4 Naphthalene sensing with Pt/PPyNQS (Impedimetry)

The naphthalene stock solution of 0.001 M was prepared by dissolving 0.0128 g in

about 5 cm3 of acetonitrile (to achieve solubility) and later made up to 100 cm3

solution in distilled water with continuous stirring for about 30 min. From this stock

solution, aliquots (in μL) were drawn and added to the cell and used for sensor

development. Proper interaction of the analyte with the polymer surface was achieved

by a gentle flow of argon being used for purging.

Impedance measurements

The procedure adopted for the impedimetric detection of naphthalene was as

described for benzidine in 3.4.2.

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131

Chapter Four

Results and Discussion 1

Morphology, Spectroscopy, Electrochemistry and Application of

nanostructured polypyrrole-β-naphthalene sulphonic acid (PPyNSA)

composites

This chapter presents the results obtained on the chemical and electrochemical

preparation, characterization and application of polypyrrole modified with β-

naphthalene sulphonic acid (PPyNSA). The optimal synthesis conditions,

spectroscopy and electrochemistry of the NSA-doped polypyrrole nanotubes,

nanosheets and nanomicelles are discussed. The kinetics of the charge transfer

processes, the redox properties of the polymer and its suitability for as a chemical

sensor for phenol detection were also presented.

4.0 Introduction

The production of different kinds of conducting polymers in the form of nanotubes or

nanowires from metallic or organic constituents inside the void spaces of nanoporous

host material have received a lot of attention as a means of boosting electrical

properties of intrinsically conducting polymers [1-10]. Since the initial work by Wei

et al. [3] on the formation and self-assembly of Polyaniline (PANi) micro/nanotubes

by the use of surfactants as against the hitherto ‘template synthesis’ method, various

investigation have been made on the use of various surfactants for production of

nano-structured materials of polypyrrole and other conducting polymers [1-11]. Yang

(2002) [1] reported the production of self assembled micro and nanotubes of

conducting PPy using protonic organic acid dopants and proposed that dopant

micelles or pyrrole/dopant clusters act as template in the formation of the polymers

micro/nanotubes.

132

Kassim et al. [2] also reported that a higher conductivity was obtained from camphor

sulphonate doped PPY electrochemically synthesized at optimum temperature of 10 –

30 oC than those prepared at higher temperatures. The physical properties were

equally reported to be changing with the preparation temperature. Smooth and

coherent films were obtained at lower temperatures while rough and wrinkled surface

was found at higher temperatures when examined under a scanning electron

microscope (SEM).

Polypyrrole synthesis and electro-activity is affected by a number of experimental

conditions such as type of solvent, electrolyte concentration and type of electrode

material, current density, applied potential, polymerization time and temperature. The

optimization of these parameters to get nanostructured and reasonably stable PPy in

gaseous (air) and in aqueous media determines its potential application as electrode

materials for biosensors and chemical sensors amongst other applications [13-19].

The neutral form of polypyrrole is usually insulating but this can be switched

electrochemically into the conductive state by oxidative (p) or reductive (n) doping of

the monomer. The property of polypyrrole to switch reversibly from one oxidation

state to the other has led to a number of technological applications, including

secondary rechargeable batteries, fuel cells, chemical sensors, controlled drug

delivery, electrochromic and corrosion protection [11–19]. It also shows promise in

the remediation of electro-active pollutants and lot of environmental control

applications [20].

Despite the easy polymerization of pyrrole and the extensive π-conjugated polymer

chain of PPY, the application is often limited by its insolubility in aqueous and

organic solvents. Various efforts have been made to add or remove electrons from the

polymer’s backbone to generate a conductive state often described as doped PPy. The

use of bulky sulphonated organic acid dopants has been proposed as a means of

generating the ionic form (PPy+A-)x (Fig. 4.1) in which polypyrrole’s solubility is

enhanced [11], where A- is the bulky sulphonated organic acid anion and ‘x’ is the

number of pyrrole trimers constituting the polymer sheet. The dopant ion hosted by

the polymer improves the solubility of PPy in organic solvents. This is achieved by

the reduction of the inter- and intra-molecular interactions of the polymer chains by

the incorporation of the bulky polynuclear protonic organic acid surfactant [11].

133

Figure 4.1: Scheme for the ionic form of β–naphthalene sulphonic acid doped

polypyrrole.

The results obtained on the nanostructured polypyrrole-β-naphthalene sulphonic acid

(PPyNSA) composites prepared in this study is presented and discussed below.

4.1 Polymerization yield of PPyNSA

Preliminary investigation to ascertain the optimum conditions for the chemical

synthesis of PPyNSA was carried out at room temperature of 25 oC and at 0 – 5 oC.

The polymerisation yield obtained is as shown in Table 4.1 for polypyrrole doped

with NSA at dopant to monomer mole concentration ratios (d/m) of 1, 0.8, 0.5 and 0

in which no dopant is involved. In all cases an oxidant to monomer mole

concentration ratio (d/m) of 0.2 was used. There is a trend of higher yield from the

synthesis carried out at lower temperature with the doped system having more yield.

Table 4.1: Yield/Temperature profile for doped and undoped PPy [21]

Sample d/m mole

concentration ratio

used

Yield of

polypyrrole (g/ml)

at 25 oC

Yield of

polypyrrole (g/ml)

at 0 - 5 oC

PPyNSA 1 1.0 33 42

PPyNSA 2 0.8 35 44

PPyNSA 3 0.5 27 32

PPyDW 0 18 20

134

The optimum PPy yield of 44% was obtained for a d/m ratio of 0.8 while the lowest

yield of 18% was obtained for the undoped polymer prepared at room temperature.

This higher yield pattern observed for the doped system is due to the ability of NSA to

generate enough radicals through protonation of the pyrrole monomers that initiates

the cationic polymerisation of PPy. The NSA equally forms micelles in aqueous

solution using its hydrophilic sulphonic acid group to combine with the hydrophobic

pyrrole monomers acting as templates in the formation of micro/nanotubes [1]. The

result shows that the efficiency of the polymerization reaction is highest at the d/m

ratio of 0.8. Lee et al. [4] had reported optimum synthesis with an apparent PPy yield

of 42% for a d/m ratio of 0.5 using dodecyl benzene sulphonic acid (DBSA) as dopant

and a synthesis temperature of 00C for the same polymerization time of 24 hours.

These results are quite comparable. The lower yield value of 32% for PPyNSA-2

when a d/m ratio of 0.5 was used in comparison with the yield of 42% for PPyDBSA

from similar d/m ratio of 0.5 cited above might be due to the higher steric hindrance

associated with the bulkier binuclear naphthalene moiety compared to the

mononuclear DBSA group.

Optimisation of chemical synthesis conditions

In order to further investigate the effect of processing conditions on the preparation of

NSA-doped polypyrrole, a series of experiments were performed with 0.1M pyrrole

using different mole-concentration ratios of dopants (HCl or NSA) and oxidant

(APS). The effect of reaction medium (pH, heating, electrolyte concentration etc.) on

polymerization yield was investigated for polypyrroles prepared in oxidant alone

(PPyDW), HCl and oxidant (PPyHCl), and NSA and oxidant (PPyNSA) at different

synthesis conditions (Table 4.2). A higher yield of polypyrrole is obtainable at low

temperature synthesis of 0 – 5 oC in all cases. The yield was further increased with an

initial heating of the NSA – Pyrrole solutions at 50 – 60 oC for 15 minutes and

cooling before the addition of the oxidant than when there was no heating. This

preparation procedure enhanced the production of higher concentration of pyrrole

complexed with naphthalene sulphonate ion (Py+NSA-) that was subsequently

oxidized to form dimeric radical cations and radical species undergoing slow cross

linking as polymerization progressed in line with the mechanism in Fig. 4.2.

135

N

H

N

H

+ H+ + A-

N

H

A-+H

+

A-

+ H+-e

N

H

+ N

H

+

A-

N

H

N

H

+

A-

Electrodepyrrole complex (a) radical cation

-H+H

N

H

N

H(b) radical specie

+ -e N

H

N

H

N

H

+

(b) radical specie

H

NN

H

+ A-

N

H

+ + A-

(c) radical cation

2H+

Figure 4.2: Scheme for the polymerization of polypyrrole via pyrrole-complex

intermediates using the radical cations and the radical species that are free of

naphthalene sulphonate specie [11].

The effect of o/m mole concentration ratio on polypyrrole chemical synthesis was

explored by using APS – monomer concentration ratio greater than the 0.2 always

reported for PPy [4, 5]. In this work, synthesis using an APS and pyrrole (o/m) of 1.0,

as normally applied in polyaniline chemical synthesis [3] produced higher yields of

PPyNSA: 98% at room temperature and 99% at 0 – 5 oC. Similar high polymer yields

were reported for polythiophene prepared with FeCl3 and thiophene in CHCl3 medium

[13]. The trend in PPyNSA yields in Table 4.2 could be attributed to greater

interactions between the dopants and monomer species leading to formation of higher

molecular weight polymers at higher concentration of APS than from lower

concentration of APS [4].

The role of dopant concentration and its anionic size in the polymerization process is

demonstrated in the yield results for APS only, APS with HCl, and APS with NSA.

The results in Table 4.2 showed that a higher yield of PPyHCl was obtained with a

136

NSA – hydrochloric acid concentrations (d/m) ratio of 0.5 than with 0.8, and in case

of PPyNSA, a higher yield was obtained from NSA – Pyrrole concentrations ratio of

0.8 than 0.5 or 1.0. This trend indicates that an optimal acid concentration is required

for the various systems.

Table 4.2: Yield of various polypyrroles prepared under different synthesis

conditions [22].

Sample d/m mixture

pre-heating

*d/m

ratio

*o/m

ratio

pH of mixture %yield (m/v)

(25 oC)

%yield (m/v)

(0 oC)

PPyDW no 0 0.2 1.8 18 20

PPyHCL yes 0.1 0.2 1.7 26 27

PPyHCL yes 0.5 0.2 0.5 *nd 47

PPyHCL yes 0.8 0.2 0.2 *nd 27

PPyNSA no 1.0 0.2 1.4 33 42

PPyNSA no 0.8 0.2 1.5 35 44

PPyNSA no 0.5 0.2 1.7 27 32

PPyNSA yes 0.5 0.2 1.7 *nd 39

PPyNSA yes 0.8 0.2 1.6 37 55

PPyNSA yes 0.8 0.5 1.4 *nd 67

PPyNSA yes 0.8 1.0 1.2 98 99

• * defined as follows:‘d/m’ is mole concentration ratios of dopant (NSA) to

monomer (pyrrole); ‘o/m’ is mole concentration ratios of oxidant (APS) to

monomer (pyrrole); and ‘nd’ are values non-determined values.

The various synthesis involved use of acidic medium with pH of less than 2. The yield

of polypyrrole is optimal at a pH of about 1.0 – 1.5 while it is limited at very low pH.

Yakovleva [11] reported that agitating the dopant and monomer mixture at pH less

than 1.0 did dramatically decelerate electrosynthesis of polypyrrole film [11]. The

137

yield of the polypyrrole obtained from this study was found to increase with the size

of dopant ion used for doping. Earlier work on electrochemically-synthesized

polypyrrole had posited that the yield increases with the size of dopant ion used [11].

For example, the rate of electropolymerisation of polypyrrole in different electrolytes

follow the trend ClO4- > HSO4

- > Cl-. In this study, the yield follows the trend

PPyNSA >PPyHCl >PPyDW in favour of larger ionic dopants.

4.2 Morphology of PPyNSA

The physical appearance of PPyNSA obtained at 0 – 5 oC was smooth and shiny while

those prepared at higher temperature appear rough and loose. This is in line with the

findings of Kassim et al. that the rough physical appearance was due to greater rates

of polymerization at higher temperatures that leads to faster nucleation and

precipitation of nanoparticles [2]. However, even when higher yields are obtainable at

higher temperatures, there is higher overlapping of micelles, which leads to surface

roughness; unlike ordered and smother nodule morphology observed at lower

temperatures. From the scanning electron micrographs (Fig. 4.3) taken at

magnification of 500 for PPyNSA doped in the ratio 1, 0.8 and 0.5 respectively; the

trends posited above could be confirmed. The lumps (side-growth) at the sides of the

identified micro/nanotubes are products from overlapped micelles, which is more

probable at higher temperatures. The micrographs of similarly doped polyaniline

(PANi) with NSA at 0 oC have very few lumps but more of tubes and some few flakes

[3]. Wei et al. had posited an optimum d/m mole concentration ratio of 0.5 for

PANiNSA [3]. At this ratio, an excess concentration of aniline is available to form

NSA micelles with aniline to form nanotubes, whereas at higher ratios lesser moles of

aniline are available for polymerization and bigger tubes were obtained. However, at

lower d/m ratios, smaller diameter nanofibre/tubes were obtained. When the

surfactant, NSA, was not used the polypyrrole featured folded sheets of fibers with

tube-like morphology (Fig 4.4). The sheet-like fibers rolled up to form tubes with a

hollow inner cavity. In this synthesis, APS produced both the required acid medium

for the synthesis and peroxodisulfate counter ion, which served as dopant. The

micrographs observed from different sides of the PPyDW could be attributed to lack

of a rigid template for the formation of compact nanostructure. In alkaline media, thin

138

insulating films are formed from the electrochemically synthesized PPy [6]. The

PPyDW synthesized from the less acidic medium of oxidant and monomer will be

less conducting. The stability of the conducting cationic polypyrrole intermediate in

the polymerization process is favoured in acidic medium.

(a) (b) (c)

Figure 4.3: Typical SEM images of different polypyrrole synthesized with o/m

0.2 at 25 oC (a) PPyNSA nano/microtubes and fibers from d/m 1, (b) PPyNSA

nano/microtubes from d/m 0.8, and (c) PPyNSA microfibers from d/m 0.8 [21].

.

(a) (b) (c)

Figure 4.4: Typical SEM images of polypyrrole nano/micro sheets synthesized

with o/m 0.2 at 25 oC in the absence of NSA dopant (PPyDW) [21] taken from

different sides to show the sheet-like fibrous structures (a) hollow tubular tubes

(b) and the rod-like microstructures (c).

139

Morphology of polypyrrole under optimum synthesis conditions

The SEM micrographs for PPyDW and PPyNSA synthesized under different

conditions showed different morphologies including thin sheets, fibres, micro

rods/tubes, nano-micelles and nano-rods/tubes (Fig. 4.5). The diameter of the tubules

ranged from 150 - 3000 nm, while the sheets have a diameter of 20 nm with lengths in

excess of 120 μm. More tubes were observed when heating was involved. The use of

higher ratios of APS - Pyrrole mole concentrations gave honeycombed clews with

smooth fibrillar surface. Figure 4.5(b), which was taken under high magnification

revealed aggregations of nanostructures that appeared like micelles with fairly

uniform diameter of ca. 100 – 150 nm. These are compactly joined together in chains

that would afford good electron hopping. No wonder the unique electrochemistry seen

during the kinetic investigation. The inter-connected porous morphology of the

PPyNSA prepared from a higher ratio of o/m is different from those of PPyNSA,

PPyHCl and PPyDW prepared from lower o/m ratios. The folded sheets (Fig. 4.5c)

from the APS doped polypyrrole (PPyDW) have lengths of about 15 - 50 μm. The rate

of polymerisation did not allow the formation of tubes, rather a highly dispersed

polymer with open fibril morphology [18]. Apart from the micro/nanoscopic

structures seen in the polymer micrographs, it is interesting to note that synthesis

performed at low temperature of 0 oC produced polymers with smooth and coherent

physical outlook, while those prepared at room temperature were coarse and rough.

Kassim et al. [2] attributed this peculiarity to α – β and β – β chain bonding of the

monomers during polymerization at the higher temperature, as against α – α chain

bonding in low temperature synthesis.

140

(a) (b)

(c)

Figure 4.5: Typical SEM images of different polypyrrole synthesized at 0 oC (a)

nano/microtubes and fibers from PPyNSA (d/m 0.8; o/m 0.2), (b) nanomicelles

from PPyNSA (d/m 0.8; o/m 1.0), (c) nanosheets from PPyDW (o/m 0.2) [22].

141

4.3 UV-Vis absorption and solubility of PPyNSA

The various PPyNSA’s were sparingly soluble in m- cresol, chloroform and dimethyl

sulfoxide (DMSO). The polymer was found to be more soluble in DMSO than in

other solvents. The PPyNSA prepared by pre-heating of the dopant /monomer mixture

had higher solubility and more intense coloration than those prepared without

preheating. Similarly, synthesis at 0 oC produced PPy with higher solubility and

spectroscopic absorption than PPy prepared at room temperature synthesis. PPyHCl

showed the least solubility in all solvents.

From the UV-Vis spectra (Fig. 4.6), the absorption maxima for the conjugated double

bonds (i.e. π to π* transition) for both PPyDW and PPyHCl were seen at 291 nm.

However, PPyNSA (d/m 0.5 and o/m 0.2) has an absorption maximum at 300 nm.

This red shift is an indication of involvement of the bulky dopant ion (NSA) in the

polymerization process [23]. From Fig. 4.6A, there are prominent polaron band at 437

nm; bipolaron band at 555 nm and over-lapped bipolaron band at 765 nm upwards for

the PPyNSA prepared from NSA - pyrrole concentration ratios of 0.5 and APS –

pyrrole concentration ratios of 0.2. The PPyNSA prepared from NSA - pyrrole

concentration ratio of 0.8 and APS - Pyrrole concentration ratio of 0.2, 0.5 and 1.0

gave only the polaron band at 409 nm (Fig. 4.6B). The concentration of polarons and

bipolaron ions along the polymer chain must have contributed to the improved

solubility of the polymer in DMSO. This is evident in the higher polaronic absorption

(437 – 765 nm) for PPyNSA prepared from NSA - pyrrole concentration ratios of 0.5

and APS - pyrrole concentration ratios of 0.2.

142

(A)

(B)

Figure 4.6: UV-Vis results for PPyNSA prepared under different synthesis

conditions: Fig. 4.6A. PPyNSA (d/m, o/m): (a) [0.5, 0.2] and (b) [0.8, 0.2], Fig.

4.6B. PPyNSA [d/m 0.8] from o/m ratios: (a) 0.2, (b) 0.5 and (c) 1.0 [22].

143

Previous work on DBSA-doped polypyrrole by Lee et al. showed bi-polaron

absorption at 480 nm and free carrier tailing in the near IR region from 750 – 1500

nm region [4]. Diffused reflectance UV-Vis-NIR spectra of doped polypyrrole by

Geetha and Trivedi [24] gave indications of cation radicals at wavelength band of 400

– 600 nm for doped polypyrrole and other bands at above 800 nm associated to

trapped excitons (bipolarons). Similarly, Shiigi et al. reported that the polaron and bi-

polaron absorption peaks at 420 nm and 860 nm respectively for overoxidised

polypyrrole colloids are greatly dependent on the concentration and type of oxidant

used [9]. It could therefore be postulated that the formation of the bipolaron band at

555 nm and over-lapped bipolaron bands at 765 nm upwards for PPyNSA system is

feasible at an optimum concentration of d/m 0.5 and o/m 0.2 using (NH4)2S2O8 as

oxidant.

It is note worthy to mention that the PPyNSA polymer with 99% yield from a d/m of

0.8 and o/m of 1.0 did not give sharp indication of trapped polaron and bipolaron (Fig.

4.6B) within the wavelength investigated. This might be due to the facile oxidation

process, which did not allow for ordered entrapment of the charge carriers. However,

the polaron peak was more prominent when an o/m concentration ratio of 1.0 was

used as against when 0.5 was used. The increasing order of oxidation power of some

common oxidants is K3Fe(CN)6 < FeCl3 < (NH4)2S2O8. The level of chemical

oxidation could therefore be used to modulate the applicability of PPyNSA. High

performance polymeric nanosensors should exhibit polaron and bipolaron bands as

obtained for the PPyNSA prepared from of NSA – pyrrole concentration ratios of 0.5

and APS – pyrrole concentration ratios of 0.2 in Fig. 4.6A. The combination of other

techniques is therefore necessary to further explore the properties of these polymers

[15].

4.4 FTIR spectral studies

Figure 4.7 shows the IR spectra of (a) PPyDW, (b) PPyNSA (d/m 0.8; o/m 0.2) and

(c) PPyNSA (d/m 0.8; o/m 1.0). Principal absorption bands observed are given in

Table 4.3 together with those reported for polypyrrole by Geetha and Trivedi [24]. All

the characteristic IR bands for polypyrrole were observed with slight variation in the

absolute values of the absorption bands. The usual N – H stretching at 3400 cm-1 in

144

neutral polypyrrole is virtually absent in all the three samples indicating that the

polymer exists in doped state in PPyDW, PPyNSA (d/m 0.8, o/m 0.2) and PPyNSA

(d/m 0.8, o/m 1.0).

Figure 4.7: FTIR spectra of polypyrroles in KBr medium for: (a) PPyDW, (b)

PPyNSA (d/m 0.8; o/m 1.0) and (c) PPyNSA (d/m 0.8; o/m 0.2) [22].

145

Table 4.3: Major shifts of bands (cm-1) in FTIR spectra of PPyDW, PPyNSA

(d/m 0.8, o/m 0.2) and PPyNSA (d/m 0.8, o/m 1.0) from undoped polypyrrole

major bands (Geetha & Trivedi) [24].

Major bands (cm-1) PPYDW PPYNSA PPYNSA

(d/m 0.8, o/m 0.2) (d/m 0.8, o/m 1.0).

3421 (N-H str) absent absent absent

3100 (C-H str) absent absent 3195 (sharp)

1535 (C=C &

C-C str) 1554 1564 1550

1450 (N-H str) absent 1461 1400

1295 (C-H &

N-H def) 1308 1310 1299

1050 (C-H def) 1047 1048 1045

The N – H stretching band for neutral polypyrrole at 1450 cm-1 was totally absent in

PPyDW but shifted positively by 11 cm-1 in PPyNSA (d/m 0.8, o/m 0.2) and

negatively by 50 cm-1 in the most oxidized polypyrrole PPyNSA (d/m 0.8, o/m 1.0).

This is an indication that the polymer is not in the aromatic state but in the excited

polaron and bipolaron defect state. The strong electronic excitations occasioned by the

polypyrrole-oxidized states strongly distort the observed absorptions. The C=C and C-

C stretching band at 1535 cm-1 shifted positively by 15 – 24 cm-1 in the three scenario

investigated and is synonymous to the absorption band of 1545 cm-1 observed for

doped PPy [24]. Similarly, the C-H stretching band at 3100 cm-1 was absent in all the

three oxidized polypyrrole, though there was a new sharp and pronounced peak at

3190 cm-1 for PPyNSA (d/m 0.8, o/m 1.0). However, the C-H deformation at 1050

cm-1 was observed at 1047, 1048 and 1045 cm-1 for PPyDW, PPyNSA (d/m 0.8; o/m

0.2) and PPyNSA (d/m 0.8; o/m 1.0) respectively. The new sharp peak observed at

3204 cm-1 in the PPyNSA (d/m 0.8, o/m 1.0) spectrum might be due to hydrogen

146

bonded N – H or O – H bond. This is synonymous to similar sharp peak at about 3500

cm-1 for the undoped polypyrrole spectra by Geetha and Trivedi [24]. Similar

anomalous trend was seen in the UV-Vis spectra of the polymer (Fig. 4.7b), which did

not show major absorption for the existence of over-lapped cationic species (bi-

polarons) for the polymer at the wavelength investigated.

The relative oxidation of the polymers can be measured from the relative intensities of

the band at 2340 cm-1. Rodriquez et al [25] reported the possible disruption of the

electronic conduction of the polymer by irreversible oxidation, leading to the

formation of carbonyl groups (1720 cm-1) and invariably CO2 (2340 cm-1). The

observed band at 2340 cm-1 in this study (Fig 4.7) was most prominent in PPyNSA

(d/m 0.8, o/m 1.0), while the intensity is very low in PPyDW and virtually absent in

the PPyNSA (d/m 0.8, o/m 0.2).

4.5 Electrochemical studies on PPyHCl and PPyNSA

4.5.1 Chemically synthesised PPyNSA

4.5.1.1 Voltammetric studies of GCE/PPyNSA systems

Cyclic voltammetry

Cyclic voltammetry results of the PPyNSA prepared at 0 oC from NSA-pyrrole mole

ratio of 0.8, indicated the existence of two redox couples a/a' and b/b'. The multiscan

voltammogram is shown in Figure 4.8 for PPyNSA (d/m 0.8, o/m 0.2) and PPyNSA

(d/m 0.8, o/m 1.0). These two redox couples gradually turns to single couple at scan

rates above 50 mV/s. In proposing a mechanism for the polymerization of polypyrrole

in aqueous medium, Yakovleva [11] predicted the probability of the formation of

pyrrole complexes in acid solutions with the proton and anion in solution, in

accordance with the scheme in Fig. 4.2. He opined that the discharge of pyrrole

complexes with the anion and proton and their destruction yield radical-cation and

radical species which could jointly interact with the active ends of pyrrole links to

induce the growth and development of polymer chains. According to a scheme

proposed by Yakovleva [11] (Fig. 4.2), the first anodic peak at ‘a’ represents the

147

oxidation of the neutral PPyNSA to the PPyNSA intermediate radical cation. This

is further oxidized at a higher potential to PPyNSA radical cation at ‘b’. The

cathodic voltammogram shows the reduction of PPyNSA radical cation to the partly

reduced PPyNSA radical anion at ‘b'’ which is further reduced to the neutral

PPyNSA at a'.

The voltammograms in Figure 4.8 show that as the potential scan rate increases, the

anodic peak potentials (Ep,a) shifts positively, the cathodic peak potentials (Ep,c)

remained unchanged and all peak currents (Ip) increase progressively. The

independence of Ep,c on scan rate is characteristic of surface-bound thin film

electroactive species undergoing fast electron transfer reaction at the electrode. For

both redox couples a/a' and b/b', the peak separations ∆Ep (i.e Ep,a – Ep,c) is < 65

mV even at higher scan rates (results for higher scan rates are not shown) which

indicates that the shift in Ep,a values with scan rate is due to intra-molecular charge

transportation. Similar peaks characterize the voltammogram of polyaniline in 1M

HCl [26–29].

148

(a)

(b)

Figure 4.8: (a) Multi–scan rate voltammograms in 0.1 M HCl for (a) PPyNSA

(d/m 0.8; o/m 0.2) at scan rates of 10, 20, 30 mVs-1; (b) PPyNSA (d/m 0.8; o/m

1.0) at scan rates of 5, 10, 20 mVs-1 in 0.1M HCl [22].

149

Square wave and differential pulse voltammetry

The low frequency (2, 3, 4 and 5 Hz) square wave voltammograms (SWV) of

GCE/PPyNSA is presented in Fig 4.9. The voltammograms (Fig. 4.9 a and b) show

two redox couples (a/a' and b/b') that are close, broad and coupling at a formal

potential (Eo') of about 241±7 mV vrs. Ag/AgCl. This agrees with the estimated Eo'

value of 241±11 mV calculated for the CV of PPyNSA (d/m 0.8; o/m 1.0) (Fig. 4.8b).

The corresponding formal potentials for the emerging peak from the convolution of

the two-redox couples are shown in Fig 4.9b. The SWV data show that the separation

between the peak potentials (∆Ep) of redox couples a/a' and b/b' has a value of about

112 mV, which must have arisen from the overlapping of two closely related 1-

electron redox processes. Similar trend was observed for the PPyNSA (d/m 0.8; o/m

0.2) shown in Figure 4.8a. The anodic differential pulse voltammograms (DPV) of the

chemically doped PPyNSA [d/m 0.8, o/m 1] at low scan rates of 5 mV/s and 10 mV/s

are presented in Fig. 4.10. The anodic peaks show a convolution of two-merging

peaks at 195 mV and 265 mV showing that two anodic peaks may be involved. These

peak potentials are close to the anodic peak points obtained for the cyclic and square

wave voltammograms in Fig. 4 .8 and 4.9. Similar trend was observed on the

PPyNSA [d/m 0.8, o/m 0.2].

150

(a)

(b)

Figure 4.9: Typical anodic SWV of PPyNSA (d/m 0.8; o/m 1.0) using an

amplitude of 50 mV and at frequencies of 2, 3, 4 and 5 Hz showing (a) the

forward and reverse waves, (b) the net square wave responses.

151

Figure 4.10: Typical anodic DPV of PPyNSA (d/m 0.8; o/m 1.0) at scan rates of 5

and 10 mV/s using amplitude of 25 mV.

4.5.1.2 Electrode kinetics of GCE/PPyNSA systems

The cyclic voltammograms of PPyNSA (Fig. 4.8) show that both the anodic and

cathodic peak currents did not come to zero at the switch potentials, indicating that

the polymer is neither over-oxidized nor over-reduced during potential cycling. At the

potential window of -500 mV to 1000 mV, where the PPyNSA (d/m 0.8; o/m 1.0)

(Fig. 4.8b) is characterised, the Eo' value for couple a/a' is 184±9 mV while that of

couple b/b' is 298±12 mV within the scan rates of 5 mV to 30 mV. The mid point, i.e.,

1/2(Eo'a/a' - Eo'b/b'), of the two redox couples, a/a' and b/b', occur at 241±11 mV.

Kinetic evaluations based on data from the CV obtained at scan rates of 5 mV to 100

mV were used to study the electrode processes of the polymer systems. A summary of

152

the kinetic evaluation discussed underneath are summarised in Tables 4.4 for

PPyNSA (d/m 0.8; o/m 1.0) and Table 4.5 for the PPyNSA (d/m 0.8; o/m 0.2) system.

They gave peak separations of less than 60 mV at different scan rates between 5 and

100 mVs-1. In the GCE/ PPyNSA (d/m 0.8; o/m 1.0) system, which was evaluated

based on CVs at potential window of -500 to 1000 mV, the oxidation to reduction

peak current ratios |Ipa/Ipc)| range was 0.8 to 0.9 for redox couple a/a' and 1.0 to1.3

i.e. [1.23±0.08] for redox couple b/b'. The ∆Ep ranged from 3.8 to 23.4 mV i.e.

[17.5±9.0 mV] for a/a', and 11.3 to 54.1 mV i.e. [37±14 mV] for b/b'. The peak

current values show linear dependence on scan rates for the cathodic and anodic

peaks, with slopes of 1.45 to 1.87 x 10-4 A.s.V-1 and correlation coefficient (r2), of

0.997 to 0.998 for both anodic and cathodic plots. The intercept values, for the non-

faradaic currents caused by the charging of the electrical double layer were close to

zero being 4.6 x 10-6 A for the anodic plot and to 5.6 x 10-6 A for the cathodic plot.

This is an indication of a very thin layer of adsorbed electro active material at the

electrode surface undergoing near-Nernstian (quasi-reversible) reaction.

Similar trend was seen in the PPyNSA (d/m 0.8; o/m 0.2), (Fig 4.8a), studied at a

potential window of –500 mV to +500 mV at scan rates between 10 mV/s and 100

mV/s. The formal potential, Eº', was 176.3±5.4 mV with a peak separation of 48 to 61

mV i.e. [52.4±4.4 mV] and |Ipa / Ipc| of 0.98 to 1.22 i.e. [1.13±0.08] for a/a'. This is

indicative of a quasi-reversible system. The linear relationship of the peak currents

versus the scan rates featured a slope of 4.32 x 10-4 A.s.V-1 and 3.38 x 10-4 A.s.V-1 for

the anodic and cathodic plots respectively. The correlation coefficient (r2) for the

anodic and cathodic peaks is 0.999 and 0.998, while the intercepts are 3.37 x 10-6 A

and 4.36 x 10-6 A, respectively.

The surface concentration, Γ*, of the PPyNSA, was calculated from the peak currents

of the CV’s in Fig. 4.8 using the Brown-Anson method (26-29). The Γ* values were

2.60 x 10-8 mol cm-2 and 5.76 x 10-9 mol cm-2 for PPyNSA (o/m 1.0) and PPyNSA

(o/m 0.2) respectively. The results shows that Γ*PPyNSA (o/m 1.0) = 4.5 Γ*PPyNSA (o/m 0.2)

respectively. This agrees with the APS (oxidant) ratio in the two PPyNSA. As

expected the polypyrrole prepared with higher APS ratio is expected to have more

PPy.

153

The peak separations for the redox couple, a/a', of GCE/PPyNSA (d/m 0.8; o/m 0.2)

system in Fig. 4.8a increased progressively from 49 mV at 10 mVs-1 to 61 mV at 100

mVs-1 coupled with increase in the magnitude of the peak currents with increase in

scan rates. Similarly, the peak separations for the redox couple, a/a', in GCE/PPyNSA

(d/m 0.8; o/m 1.0) system (Fig.4.8b) increased progressively from 4 mV at 5 mVs-1 to

54 mV at 100 mVs-1. These trends show that the peak currents are diffusion

controlled. Thus the Randel- Sevčik equation (equation 3.6) was applied to determine

the diffusion coefficient (De) for electron hopping along the polymer chain [21].

Ip / ν1/2 = 2.686 x 105 n3/2 A Γ*PPyNQS.De

1/2

Figures 4.11 (a) and (b) represent plots of the absolute magnitude of the cathodic peak

currents vrs. square root of scan rates for GCE/PPyNSA (d/m 0.8; o/m 0.2) and

GCE/PPyNSA (d/m 0.8; o/m 1.0) systems respectively. The charge transfer

coefficient, De, of the polymers were estimated from the slopes of the graphs to be

1.81 x 10-6 cm2s-1 and 1.21 x 10-6 cm2s-1 for PPyNSA (o/m 1.0) and PPyNSA (o/m

0.2) respectively [28, 29]. Thus the PPyNSA that was prepared with higher

oxidant/monomer ratio produced polymer with an enhanced charge transportion, i.e

better conductivity (See Fig. 4.13). The De values for PPyNSA agree with the

diffusion coefficient of 1.10 x 10-6 cm2s-1 reported for chloride ions in polypyrrole

[30].

154

(a)

(b)

Figure 4.11: Plot showing the variation of the modulus of both the anodic and

cathodic peak currents with square root of scan rates for (a) GCE/PPyNSA (d/m

0.8; o/m 0.2) system and (b) GCE/PPyNSA (d/m 0.8; o/m 1.0) in 0.1 M HCl.

155

The standard rate constant, kº, i.e. kº = φ (α. n. F. ν. De / RT) ½ [31-33] for the

electron transfer reaction of the polymer on GCE was calculated using the Nicholson

method. The φ (dimensionless parameter) value of 20 was obtained based on a peak

separation, ΔEp, value of 61.1 mV [31, 33] for the voltammogram at 100 mVs-1,

which gives the number of electron transferred, n, as 1. Assuming a transfer

coefficient, α value of 0.5, the kº values were calculated as 3.75 x 10-2 cms-1 and 3.08

x 10-2 cms-1 for PPyNSA (o/m 1.0) and PPyNSA (o/m 0.2) respectively. This is

comparable with the rate constants reported for other conducting polymers, vis.:

Pt/PANI electrodes with kº values of 0.049 to 5.4 x 10-3 cms-1 [34] and Pt/PEDOT

electrodes with kº values of 1.5 to 45.3 x 10-3 cms-1 [35] under different synthesis

conditions.

The conductivity in the polypyrrole film on the electrode surface arises from the

electronic transfer along the conjugated л-molecular orbital coupled with the motion

of charge carriers in the material. When electron is removed from the л-system of the

PPy backbone, a polaronic radical cation is produced by the local distortion of charge

over four pyrrole units. Further oxidation of the polaronic radicals leads to formation

of the bipolarons, which are energetically more favourable [36]. The final

conductivity of the polymer is determined by the combination of two processes

namely; charge carrier mobility (‘hopping’) along individual polypyrrole segment and

the charge transfer between the dopant and the polymer segment. More research on

fuller understanding of the charge storage and transport mechanism in conducting

polymers is still going on [37].

156

Table 4.4. Summary of estimates of kinetic parameters for PPyNSA [d/m 0.8,

o/m 1.0] on GCE based on n = 1, and scan rate measurements from 10 – 100

mV/s.

Couple 1 (a/ a') Couple 2 (b/ b') Parameter

Anodic (a) Cathodic (a') Anodic (b) Cathodic (b')

Γ*PPyNSA 2.23 x 10-8 mol cm-2 2.54 x 10-8 mol cm-2 2.81 x 10-8 mol cm-2 2.18 x 10-8 mol cm-2

De PPyNSA 1.81 x 10-6cm2s-1 1.81 x 10-6cm2s-1 1.80 x 10-6cm2s-1 1.82 x 10-6cm2s-1

ķ º (100mV/s) 3.75 x 10-2cms-1 3.75 x 10-2cms-1

Eo' (mV)

∆Ep (mV)

Ip,a/Ip,c

187.4 299.5

3.8 – 23.4 11.3 – 54.1

0.8 – 0.9 1.05 – 1.27

Table 4.5. Summary of estimates of kinetic parameters for PPyNSA [d/m 0.8,

o/m 0.2] on GCE based on n = 1, and scan rate measurements from 10 – 100

mV/s. NB: The second couple at Eº (mV) of 275 mV, was not clearly resolved at

all scan rates.

Couple Average Parameter

Anionic (a) Cathodic (a')

Γ*PPyNSA 6.43 x 10-9 mol cm-2 5.09 x 10-9 mol cm-2 5.76 x 10-9 mol cm-2

De PPyNSA 1.22 x 10-6cm2s-1 1.21 x 10-6cm2s-1 1.22 x 10-6cm2s-1

ķ º (100mV/s)

Eo' (mV)

∆Ep (mV)

Ip,a/Ip,c

3.08 x 10-2cms-1

162.55

48.9 -61.1

0.98 – 1.22

157

4.5.1.3 Impedance studies PPyNSA systems

Electrochemical impedance spectroscopy (EIS) interrogation of PPyNSA were

performed with an ac amplitude of 10 mV for potentials -600 to 1000 mV over a

frequency range from 105 to 10-1 Hz. The behaviours of the of PPyNSA (d/m 0.8, o/m

1.0) system at different potentials in terms of conductivity vis-à-vis its charge storage

capacity at a low frequency of 10-1 Hz is presented in Fig. 4.12. The potentials at

which the polymer is most conductive were 100 – 300 mV with a real impedance

value of about 0.30 – 0.40 kΩ cm2 based on the unfitted data. Coincidentally, the

highest charge storage ability of 0.73 - 0.95 mF cm-2 was displayed at these potentials

with the highest value of 0.95 mF cm-2 observed at 300 mV (Fig. 4.12). Low

capacitance trend of ≤ 0.2 mF cm-2 were manifested at potentials less than -200 mV

within the potential window investigated. Similar behaviour was seen with the

PPyNSA (d/m 0.8, o/m o.2) system even though it showed lower trend of conductivity

as could be seen from the higher impedance values in Fig. 4.13. The trend of

conductivity for the three system at the low frequency value of 10-1 Hz shows that

PPyNSA (o/m 1.0) > PPyNSA (o/m 0.2) > PPyDW (Fig. 4.13). This confirms our

position that the NSA-doped polypyrrole from higher oxidant to monomer ratio offers

better charge transportation than that from a lower oxidant to monomer ratio. The

NSA doped polypyrrole shows maximum electroactivity at lower potential (100 – 300

mV) than the polypyrrole prepared from NSA- free medium, which shows maximum

charge transportation at about 400 – 600 mV (Fig. 4.13). The PPyNSA system is

therefore a better electrocatalyst than the PPyDW.

158

Figure 4.12: Plots of the real impedance (Z) and capacitance (C) data of PPyNSA

(d/m 0.8, o/m 1.0) system showing low frequency behaviours between -600 mV to

700 mV [22].

159

Figure 4.13: Plots of the real impedance (Z) data obtained at 0.1 Hz for PPyNSA

(d/m 0.8, o/m 1.0), PPyNSA (d/m 0.8, o/m 0.2) and PPyDW at different potentials

[22].

4.5.2 Electrosynthesised PPyHCl and PPyNSA

The electrosynthesis of the polypyrrole was first done in an electrolyte medium of

hydrochloric acid (pH, 1.1) using glassy carbon and platinum electrodes.

Subsequently, the electrosynthesis was explored with the introduction of naphthalene

sulphonic acid into the electrolyte (pH, 1.6) as described in chapter 3.

4.5.2.1 Voltammetric studies on electrosynthesised PPyHCl and PPyNSA

Electropolymerisation of PPyHCl

The voltammogramms for the electrosynthesis and characterisation of PPyHCl at

room temperature on platinum disc electrode is presented in Fig. 4.14. The

polymerization wave gave sharp oxidation and reduction peaks. The PPyHCl film

grown on GCE does not give similar sharp peaks but rather forming plateaus at the

160

maximum anodic and cathodic currents (Fig. 4.15.). The voltammograms in Fig. 4.14

(a) and (b) showed that the Pt/PPyHCl polymer film is fully in the reduced form at

about –300mV and in the fully oxidized form at the switching potential of 600mV,

whereas the GCE/PPyHCl was not fully in the reduced form even at the switching

potential of –600mV, even though at 600mV it is fully oxidized and overoxidation

setting in. This is an attestation to the different electrochemistry of polypyrrole at

different electrode surfaces. The complete switching of polypyrrole from the fully

reduced state to the fully oxidised state is a major advantage derivable from the

Pt/PPyHCl over GCE/PPyHCl system.

(a)

161

(b)

Figure 4.14: (a) The polymerization voltammograms of Pt/PPyHCl (20 cycles)


voltammograms for electropolymerized Pt/PPyNSA at 10 to 50 mV/s.

162

(a)

163

(b)

Figure 4.15: (a) The polymerization voltammograms of GCE/PPyHCl (30 cycles)


voltammograms for electropolymerized GCE/PPyNSA at 10 to 100 mV/s.

The voltammograms for the polymerisation of Pt/PPyHCl and GCE/PPyHCl involves

oxidation of the monomer units to form the radical cations as described earlier in Fig.

2.3. The coupling i.e. dimerisation of the cation radicals leads to expulsion of two

hydrogen atoms from the reacting species followed by attack of the radical on a

neutral monomer during the oxidative scan. This is the rate-determining step [12].

With progressive cycling the dimers are more readily re-oxidised under the operating

conditions than the monomer owing to its stronger conjugation. The oxidation

potential for the dimer and oligomers of pyrrole has been reported to be lower than

that for the monomer. This is reflected with progressive shift in anodic peak potentials

to lower values with cycling in Fig. 4.14a. The chain growth proceeds via the addition

of a newly formed radical cation to an oligomeric one. Thus we have nucleation,

dimerisation, adsorption and polymerization at the anode while essentially we have

164

polymerisation at the cathode. The shifts of anodic peak position to lower potential

with cycling were due to the continuous ease of polymerization after the initiation of

the polymerization process. Subsequent growth of polymer layers can only take place

after the formation of conducting polymer nuclei at the electrode surface via

adsorption. Electrochemical evidence of this is the nucleation loop present at the

reduction switch-potential in the cyclic voltammogram in Fig. 4.14a.

Electropolymerisation of PPyNSA

The introduction of the surfactant (NSA) into the electrolyte medium caused a

reduction in the conductivity of the medium and thus posing greater difficulty in

electrolytic polymerisation. This trend is occasioned by organic nature of the NSA

which is less conducting than when pure HCl solution is used. The electrosynthesis of

PPyNSA on platinum disc electrode was not successful due to the high electrode

resistivity preventing initiation of polymerization process at the experimented

conditions. The observed trend in the electropolymerisation on GCE was however

better as there was less resistance to polymer adsorption on the electrode surface (Fig

4.16a). The multi scan voltammograms of the GCE/PPyNSA featured a diffusion

controlled system with peak separations ranging from 108 to 369mV while the |Ip,a /

Ip,c| ranged from 1.6 – 2.2 with lower values at low scan rates respectively (Fig.

4.16b). The GCE/PPyNSA redox peak potential points were unfortunately not sharp

at low scan rates. Sharper peaks were seen at scan rates of over 100 mVs-1 but they

were not kinetically related (Fig. 4.16b).

165

(a)

(b)

Figure 4.16: Polymerization voltammograms of GCE/PPyNSA (15 cycles) from

0.1 M Pyrrole and 0.05 M NSA in 0.1 M HCl at 50 mVs-1 (a), and multi scan rate

voltammogram for the electropolymerized GCE/PPyNSA at 5 to 400 mV/s (b).

166

Cyclic voltammetry (CV) of over-oxidized electrosynthesised PPyNSA

During the potentio-dynamic electro-synthesis of PPyNSA at room temperature in

excess of a potential higher than 800mV, I observed there was over oxidation of

PPyNSA. It had been reported that the use of platinum electrode for the synthesis of

metal-doped polypyrrole has the disadvantage of loss of polypyrrole activity at

potentials more anodic than 700mV and activity decrease at potentials more cathodic

than – 300mV [11]. This was traced to the high electro catalytic activity of

polypyrrole on platinum electrode. The electropolymerisation (10 cycles) of PPyNSA

on GCE using high anodic switch potential of 1500 mV from from 0.1 M Pyrrole and

0.05 M NSA in 0.1 M HCl using a scan rate of 20 mV/s at room temperature is shown

in Figure 4.17a. At the potential window of -800mV to 1500mV used, there was

cathodic growth over the first four cycles of the 10 cycles of polymerization after

which growth ceased. The anodic peak current showed continual decrease in peak

current with cycling. Upon characterisation of the film, there was no anodic peak

because of the overoxidation but the cathodic peak currents increased with scan rate

from 5 to 100 mV/s (Fig. 4.17b) after which there was progressive decrease in current

with increasing scan rate. The decrease in current is due to saturation of the electrode

surface with poor conducting film generated at the high potentials used for

polymerisation. The shifts in the anodic peak currents to lower potentials with

progressive cycling was due to the high potential used, which generated less

electroactive and less conducting film. Overoxidised PPy, despite its disadvantages of

decreased electroactivity, has wide electro analytical applications that utilize perm-

selectivity [9]. The overoxidation was however curtailed when the electrosynthesis

was done using an electrolyte/monomer mixture equilibrated at 0 ˚C within the same

potential window. Usage of higher scan rates for the polymerisation is undesireable as

it saturates the electrode surface faster.

167

(a)

(b)

Figure 4.17: (a) Polymerization voltammograms of GCE/PPyNSA (10 cycles)

from 0.1 M Pyrrole and 0.05 M NSA in 0.1 M HCl at 20 mVs-1 showing

overoxidation current pattern, (b) multiscan voltammograms.

168

4.5.2.2 Kinetic studies on electrosynthesised PPyHCl

Pt/PPyHCl The dynamics of electropolymerisation of PPyHCl is as shown in Fig 4.14. Analysis

of the polymerisation and the multiscan rates voltammogram displayed in Fig. 4.14 (a

and b), as well as the OSWV and DPV investigation showed that about 3 electrons

might be involved in the polymerisation process. The peak separation, ∆Ep, between

the anodic and cathodic peaks in Fig. 4.14b is about 171 mV which suggest that there

may be more than one redox couple involved in the electrochemistry as earlier

discussed for PPyNSA. The formal potential, Eº', of the PPyHCl system in the

hydrochloride electrolyte solution of 0.1 M HCl was estimated using the CV at 5

mV/s as 86.5 mV. This was arrived at using equation 3.1,

Eº' = (Ep,a + Ep,c) / 2 = (115.9 + 57.1) / 2 = 86.5 mV

From the CV for the polymerisation of Pt/PPyHCl, Fig 4.14a, the estimate for Eo' was

Eº' = (Ep,a + Ep,c) / 2 = (152 + 19)/2 = 85.6 mV.

Similarly the OSWV at 15 Hz, and square wave amplitude of 25 mV gave Eo'as 98.3

mV. Averaging the estimates of Eo' from these three approaches gave Eo'PPyHCl as

90.1±7.3 mV at the operating conditions.

The ratio of |Ip,a/Ip,c| was 1.88±0.24 over different scan rates between 5 to 50mV/s.

At higher scan rates, the ratio increases and thus reducing the quasireversibility of the

Pt/PPyHCl. Surface concentration (Γ*PPyHCl) at the electrode surface was estimated

from data generated at 5, 10, 20, 30, 40 and 50 mV/s using the Brown-Anson model

(equation 3.5) to be Γ*(anodic) of 1.32 x 10-8 mol cm-2 and Γ*(cathodic) of 5.93 x 10-9

mol cm-2. The Γ*(cathodic) value is a better indication of the concentration of the

PPyHCl film on the platinum surface as this is the form in which the polymer is

adsorbed on the electrode surface.

A plot of the peak currents versus square root of scan rates gave linear plots for both

anodic and cathodic waves. The slopes for the anodic and cathodic plots (Fig. 4.18)

was 9.4032 x 10-4 A / (V/s) ½ (r2 = 0.9793) and 4.2572 x 10-4 A / (V/s) ½ (r2 = 0.9928)

169

respectively. The diffusion coefficient (De) was estimated from the slope in the

Randles-Sevčik plot (Fig. 4.18) using equation (equation 3.6) to be 3.20 x 10–5 cm2 s-1

for the cathodic and 1.30 x 10-6 cm2 s-1 for the anodic. The average diffusion

coefficient for the PPyHCl film is estimated as 1.67 x 10-5 cm2s-1. The polymer

showed poor reversibility between the anionic and cationic species, as seen by the

|Ip,a /Ip,c| being 1.81; Γ*(anodic specie)/ Γ*(cathodic specie) being 2.23; and De

(anodic specie)/ De (cathodic specie) being 0.04. This trend attests to the poor kinetics

and lesser quasi-reversibilty of the electrosynthesised PPyHCl compared to the

chemically synthesised PPyNSA discussed earlier.

Figure 4.18: Plot showing the variation of the modulus of both the anodic and

cathodic peak currents with square root of scan rates for Pt/PPyHCl system in

0.1 M HCl.

4.6 Amperometric response of GCE/PPyNSA to phenol

The square wave responses of the GCE/PPyNSA to phenol are shown in Fig 4.19. The

analytical region (0 to 139.5 µM) of the phenol calibration plot of the electrode is the

170

Fig. 4.20 insert. The estimated electrode sensitivity was 3.1 mA.M-1 and the detection

limit was estimated from the noise to signal ratio of the SWV responses and found to

be 0.65 µM (i.e 2 x noise / sensitivity, where noise was estimated as 1 x 10-9 A). For

extended phenol concentration range, the GCE/PPyNSA electrode follow typical

hyperbolic Michaelis-Menten kinetics reaching IMax (maximum current) of 0.5µA at

phenol concentration of 333.3 µM. The apparent Michaelis-Menten constant (K′M)

was estimated as 160 µM. This K′M value is 48% of the observed maximum substrate

concentration of 333.3 µM for the PPyNSA-GCE sensor. Iwuoha et al [38] had

reported that maximum biosensor catalytic response is obtainable at ½ IMax, when the

K′M value is reached. The K′M value obtained is within the range 10-2 and 10-7 M

obtainable for enzyme catalysed reactions [39].

Figure 4.19: Graph of the square wave voltammetric response of GCE/PPyNSA

to different concentrations of phenol in 0.05 M HCl [22].

171

Figure 4.20: Calibration plots of GCE/PPyNSA sensor for phenol concentrations.

Insert is plot of the linear region. (Plots represent SWV responses at 70 mV

taken from Fig. 4.19) [22].

Conclusions

The effect of temperature, concentration of reacting species and synthesis conditions

have been identified as critical factors for yield and morphology in conducting

polymers of PPyNSA. This study showed that at optimal synthesis temperature of 0

oC, the highest yield of PPyNSA (99 %) is obtainable with d/m 0.8 and o/m 1.0. The

morphology of the polymer varies from thin sheets to fibres, micro rods/tubes and

nano micelles/rods/tubes. The diameters of the tubules range from 150 - 3000 nm

while the nano-sheets have a diameter of 20 nm. Use of APS-pyrrole mole

concentration ratio of 1.0 produced micelles with typical diameters of ca. 100 - 150

nm. The concentration of charge transfer species along the polymer chain through

doping with the NSA improved the solubility of the polypyrrole in organic solvents.

172

Besides, the use of NSA as dopant gave better electrochemistry than when only HCl

was used. PPyHCl was the least soluble of all the polypyrrole investigated. PPyNSA

(d/m 0.5 and o/m 0.2) gave the highest solubility; and its UV-Vis revealed polaron

and bipolaron concentration at 437 nm and 555 nm respectively. In

dodecylbenzenesulphonic acid doped polypyrrole the best compromise between

conductivity and solubility in m- cresol was found to be at o/m ratio of 0.13 [40].

FTIR result show better doping for PPyNSA (d/m 0.8, o/m 0.2) than for the PPyNSA

(d/m 0.8, o/m 1.0). EIS analysis together with OSWV data shows that the β-NSA

doped polypyrrole systems exhibit highest electroactivity at potentials between 200

mV and 300 mV. Test application of the GCE/PPyNSA electrode as phenol sensor

showed that PPyNSA based sensor can be used in the detection of phenol in water and

environmental samples. The dynamic linear range of the sensor showed that the

sensor is analytically useful at phenol concentrations of 2 – 10 µM and with the

extended hyperbolic curve higher concentration range of 0.65 - 139.5 µM could be

determined. This translates to phenol mass concentration of 6.7 - 1500 parts per

billion (ppb). This is within the range of phenol found in pharmaceutical industries

and refinery effluents.

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electrodes in aqueous solutions, Journal of Electroanalytical Chemistry, 489, 17–27.

36. Ateh, D.D., Navsaria, H.A. & Vadgama, P. (2006). Polypyrrole-based conducting

polymers and interactions with biological tissues, J. R. Soc. Interface, 3, 741–752

37. Papathanassiou, A.N., Grammatikakis, J., Sakellis, I., Sakkopoulos, S., Vitoratos,

E. & Dalas, E. (2005). Hopping charge transport mechanisms in conducting

polypyrrole: studying the thermal degradation of the dielectric relaxation. Appl. Phys.

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Composite Poly-L-lysine Matrix, Electroanalysis, 11(5) 367 – 373.

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39. Chang, R. (1981). Physical Chemistry with Applications to Biological Systems,

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40. Song, M. K., Kim, Y. T., Kim, B. S., Kim, J., Char, K. & Rhee, H. W. (2004).

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alkylbenzenesulphonic acids, Synth. Met., 141, 315 – 319.

178

Chapter Five

Results and discussion 2

Morphology, Spectroscopy, Electrochemistry and Applications of

novel polypyrroles-1, 2-napthaquinone-4-sulphonate (PPyNQS)

composite

This chapter gives the results for the chemical and electrochemical preparation,

characterization and application of polypyrrole modified with sodium salt of 1, 2-

napthaquinone-4-sulphonic acid (NQS). The chemical synthesis option was based on

the optimized conditions established for PPyNSA with a dopant to monomer (d/m)

mole concentration ratio of 0.8 and oxidant to monomer (o/m) mole concentration

ratios of 0.2 and 1. Yield, morphological and spectroscopic properties of the NQS-

doped polypyrrole nanomicelles is discussed. Electrochemical synthesis option was

employed for the evaluation of the redox properties and kinetics of the charge transfer

processes of the polymer. SNIFTIR spectroscopic result provided information on the

structural changes during oxidation and reduction and evidence of doping with NQS.

The test application of the PPyNQS electrode as a chemical sensor for benzidine and

naphthalene in aqueous medium is also presented.

5. Introduction

Having achieved some success in establishing optimum conditions for the synthesis of

highly electroactive, soluble and processable naphthalene sulphonic acid doped

polypyrrole PPyNSA via the chemical synthesis options [1, 2], which is one of the

major objectives of this study, it was desirable to further explore other surfactant

routes to the modification of polypyrrole. In this chapter, nanomicelles and nano-film

of novel polypyrrole 1, 2-napthaquinone-4-sulphonate (PPyNQS) was self assembled

with sodium salt of 1, 2-napthaquinone-4-sulphonic acid (NQS) and pyrrole (Py) in

179

electrolyte medium of hydrochloric acid [3]. The polymer composite in the

nanostructured and conducting state is expected to produce improved catalytic effect.

It is well known that various conducting polymers can self assemble into

nanostructures both in solutions [4] and at electrode surfaces [5]. The ability of

surfactant mediated synthesis to give different forms of nanostructures has made its

use quite versatile in the present drive for production of improved sensor materials

[6]. In using 1, 2-naphthaquinone-4-sulphonate as dopant, we expect improvement in

the solubility, processabilty and electroactivity of the modified polymer and also

create an expansion in its applications to sensor measurements. The introduction of

the two ketonic groups on the naphthalene sulphonate is expected to further improve

the coulumbic interactions required for the production of a more electroactive,

processable, ‘intelligent’, and soluble polypyrrole [7]. Within the scope of available

information at our disposal, the synthesis of modified polypyrrole using the NQS

dopant option is novel. The ionic form of the polymer is represented by Fig. 5.1.

NHNH

NH+

O

O

SO O

O

-

x

Figure 5.1: Ionic form of polypyrrole 1, 2-naphthaquinone-4-sulphonate

(PPyNQS) smart nanomaterials.

The polymer composite formed with the NQS will combine the properties of low

dimensional material, conducting polymer and that of the functional 1, 2-

naphthaquinone-4-sulphonate (PPyNQS) acting as dopant and acid electrolyte [6].

Through careful experimental control of the conditions of self assembly of this

PPyNQS smart nanomaterial on platinum working electrode and stabilizing the film

180

through mild evaporation, a Pt/PPyNQS working electrode was formed and used as

impedimetric sensor for determination of benzidine and naphthalene in aqueous

medium.

The experimental results of the morphological, spectroscopic, electrochemical and

test applications of the PPyNQS polymer are now presented.

5.1 Chemical synthesis and yield optimization

5.1.1 Yield and morphological properties of PPyNQS

Table 5.1 presents the yield result obtained from the chemical synthesis using a d/m

mole concentration ratio of 0.80 which generated instantaneous precipitation of black

polypyrrole from the black solution of pyrrole/NQS in 0.1 M HCl.

Table 5.1: Yield profile for chemically synthesised PPyNQS at different dopant

and oxidant concentration ratios.

Code pyrrole

used

NQS/dopant

used

APS used ratios pH color

change

yield

NQS

-1

0.529 mL

(7.5 x 10-3

mol)

1.540 g

(6.0 x 10-3

mol)

0.3457 g

(1.5 x 10-3

mol)

d/m

(0.80)

o/m

(0.20)

1.41 Dark brown

to black

0.1920 g

(37 %)

NQS

-2

0.529 mL

(7.5 x 10-3

mol)

1.540 g

(6.0 x 10-3

mol)

1.745 g

(7.5 x

10-3 mol)

d/m

(0.05)

o/m

(1.00)

1.43 Dark brown

to black

0.5160 g

(104 %)

NQS

-3

0.529 mL

(7.5 x 10-3

mol)

0.0982 g

(3.776 x

10-4 mol)

0.3484 g

(1.5 x

10-3 mol)

d/m

(0.05)

o/m

(0.20)

1.40 Yellowish

brown

solution to

black

0.1660 g

(31 %)

NQS

-4

0.529 mL

(7.5 x 10-3

mol)

0.0982 g

(3.776 x

10-4 mol)

1.745 g

(7.5 x

10-3 mol)

d/m

(0.05)

o/m

(1.00)

1.43 Yellowish

brown

solution to

black

0.562 g

(106 %)

181

Fig. 5.2 shows the SEM micrograph of dry powder of PPyNQS at nanometer range. It

displayed clusters of micelles and grains with diameters ranging from 50 – 150 nm,

which are compactly joined together with helical chains of about 20 nm diameters.

The cluster arrays provide large surface area for electron hopping. The existence of

this morphology is derived from the micelle guided growth process at optimum

conditions of dopant to monomer (d/m) mole concentration ratios of 0.05 and oxidant

to monomer (o/m) mole concentration ratios of 0.2 and 1 for Fig. 5.2a and b

respectively. The pyrrole molecules preferentially dissolved into the micellar

assembly of the surfactant because of the monomers hydrophobic nature and through

external nucleation generate networks of clusters [6]. This ordered arrangement led to

the micellic clusters seen in the micrograph.

The structure of the polymer might change from spherical to rod- and lamellar-shape

as the concentration of the surfactant increases. It is therefore important to ensure that

the surfactant’s critical micelle concentration (CMC) is not exceeded for production

of electroactive polymer. The optimized synthesis condition for the electrochemical

synthesis requires a maximum d/m ratio of 0.1. At higher concentrations of surfactant

the polymer is over-oxidised. The micellic clusters seen in the chemically synthesized

PPyNQS was obtainable at d/m concentration ratio of 0.05.

Fig. 5.3 shows the energy dispersive X-ray (EDX) spectra and the elemental analysis

for carbon, oxygen and sulphur based on the micrograph portion containing the

nanostructures. The proximate % elemental composition of C:89; O:5 and S:6 for

PPyNQS compares with that for PPyNSA, which was C:91; O:3 and S:3. The higher

% elemental composition S for PPyNQS is an indication of better surfactant

incorporation into the polymers structure via sulphonation of the polypyrrole moity.

Likewise the higher % elemental composition obtained for O in PPyNQS is traceable

to the ketonic group from NQS which gave additional contribution to what is

obtainable for NSA.

182

(a)

(b)

Figure 5.2: SEM micrographs of dry powder PPyNQS showing the typical

fibrous-micellic structures obtained from (a) PPyNQS [d/m 0.05; o/m 0.2] and

(b) PPyNQS [d/m 0.05; o/m 1.0].

183

Figure 5.3: SEM and EDX analysis for dry powder of PPyNQS (top) compared

with that from naphthalene sulphonic acid doped polypyrrole (PPyNSA).

5.1.2 Spectroscopic properties of PPyNQS

FTIR

Figure 5.4 shows the Fourier Transform Infra Red (FTIR) spectra of PPyNQS

obtained from the two o/m mole concentration ratios of 0.2 and 1.0 compared with

that where NQS was not used (PPyDW). The spectra showed the existence of all the

characteristic absorption bands for polypyrrole with slight variations in the absolute

values for the PPyDW, PPyNQS (d/m 0.05; o/m 0.2) and PPyNQS (d/m 0.05; o/m

1.0). The characteristic absorption bands observed were compared with those reported

by Geetha and Trivedi [8] for doped polypyrrole and presented in Table 5.2. The

absence of the usual N – H stretching vibration at 3400 cm-1 shows that the polymers

are not in the neutral state just as was observed for the PPyNSA system in chapter

four. The N – H stretching band for neutral polypyrrole at 1450 cm-1 shifted positively

by 11 cm-1 in PPyNQS (d/m 0.05, o/m 0.2) and by 8 in the more oxidized polypyrrole

PPyNQS (d/m 0.05, o/m 1.0). This is a confirmation that the polymers were not in the

%C = 89 %O = 5 %S = 6

PPYNQS

PPYNSA

%C = 91 %O = 3 %S = 3

184

aromatic states but rather in the excited polaronic states. The C=C and C-C stretching

band absorption at 1542 cm-1 in the two PPyNQS system is synonymous to the

absorption band of 1545 cm-1 characteristic of doped PPy [8]. The usual C-H

deformation at 1050 cm-1 for doped polypyrrole was observed at 1042 cm-1 for the

PPyNQS system. The sensitivity of the instrument could not resolve clearly the

doublet, asymmetric stretching of O=S=O at 1320 and 1288 cm-1 which is expected

for polypyrrole doped with sulphonic acids. More interesting investigation was sought

using SNIFTIR and the results are discussed underneath.

Wavenumber (cm-1)

1000200030004000

Tran

smitt

ance

(%)

20

40

60

80

100

a

b

c

Figure 5.4: FTIR spectra of polypyrroles in KBr medium for: (a) PPyDW, (b)

PPyNQS (d/m 0.05; o/m 0.2) and (c) PPyNQS (d/m 0.05; o/m 1.0).

185

Table 5.2: Major shifts of bands (cm-1) in FTIR spectra of PPyDW, PPyNQS

(d/m 0.05, o/m 0.2) and PPyNQS (d/m 0.05, o/m 1.0) from undoped polypyrrole


Major bands (cm-1) PPyDW PPyNQS PPyNQS

(d/m 0.05; o/m 0.2) (d/m 0.05; o/m 1.0)

3421 (N-H str) absent absent absent

3100 (C-H str) absent absent absent

1535 (C=C &

C-C str) 1554 1542 1542

1450 (N-H str) absent 1461 1458

1295 (C-H &

N-H def) 1308 1307 1307

1050 (C-H def) 1047 1042 1042

SNIFTIRS

The substractively normalized FTIR spectra showed the ability of the modified

polymer to switch from the oxidized state to the reduced state within reasonable

potentials. A shift of the baseline was evident in the spectra obtained at potentials

increasingly positive of 0mV, when the spectra were viewed over the extended scale,

covering the complete wavenumber range experimentally available i.e. 5000 cm-1 to

1000 cm-1 (Fig. 5.5). This is related to low energy transitions within the polymer and

is evidence of the conductive nature of the modified polymer. The solvent features

seen at around 3500 cm-1 were not expected to interfere with other absorptions.

During the analysis of spectral data however emphasis was placed on the absorptions

between 2400 cm-1 to 1000 cm-1, which covered the fingerprint region (1700 to 1000

cm-1) of the spectrum (Fig. 5.6). The intensity of absorption bands present in this

region is indicative of the oxidation state of the polymer

The bands present in the region of ring stretching are particularly intense due to strong

coupling between charge carriers and ring vibrational modes, facilitating movement

186

of carriers within conjugated polymer chains. These enhanced signals (IRAV bands)

are typical of doped conducting polymers. The vibrations occur as paired bands at

very close frequencies. The downward bands at 1531 and 1586 cm-1 were assigned

to C=C and C-C vibrations. The bands at 1432 and 1380 cm-1 are assigned to C=N

vibration. The band at 1294 cm-1 was assigned to N-H in plane contributions.

Evidence of the sulphonic acid presence in the doped polymer matrix was provided by

the absorption bands at 1172 and 1080 cm-1 in Fig. 5.6 assigned to the asymmetric

stretch of the O=S=O group [9-12]. An absorption band at 2340 cm-1 which increased

in intensity at potentials above 300 mV, is indicative of the presence of CO2 [9, 12,

13].

10002000300040005000Wavenumber cm-1

-20

-10

010

20Tr

ansm

ittan

ce [%

]

0 mV

600 mV

Finger print region

(enlarged)

H2O absorption

CO2

Figure 5.5: Full SNIFTIRS spectra of PPyNQS at 100 mV potential intervals

from 0 to 600 mV, vs calomel electrode.

187

Figure 5.6: Normalised SNIFTIRS spectra of PPyNQS showing the enlargement

of the finger print region.

UV-Vis results

The polymer showed good solubility in DMSO and DMF. Figure 5.7 shows the UV-

Vis of PPyNQS compared with that of polypyrrole prepared in distilled water in the

undoped state, (PPyDW) taken in DMF. The sharp peaks at 380, 750 and 820 nm

corresponds to the polaron, bipolaron and overlapped bi-polaron bands within the

polymeric structure [8, 14]. These absorptions are not associated with that of PPyDW.

The concentration of these charge carriers lends credence to the intrinsic high

electroactivity of PPyNQS through the incorporation of the surfactant dopant. The

wavelength absorption bands here are similar to that of polypyrrole doped with

naphthalene sulphonic acid in the optimized ratio of dopant/monomer (d/m)

concentration ratio of 0.5 and oxidant/monomer (o/m) concentration ratio of 0.2 [6].

188

Figure 5.7: UV-Vis results for PPyNQS compared with that of PPyDW.

5.2 Electrochemical investigation

5.2.1 Chemically synthesized PPyNQS:

Since our subsequent sensor application was based on the electrochemically generated

PPyNQS, the electrochemical investigation carried out was concentrated on the

electrochemically synthesized product.

5.2.2 Electrochemically synthesized PPyNQS

The electrosynthesis of PPyNQS in the acidic aqueous electrolyte was very successive

on both platinum and glassy carbon electrode. The choice of platinum electrode was

made because it offers a transparent surface for comparative spectroscopic studies.

Initial investigation showed that better electropolymerisation and film adherence is

obtainable at 50 mVs-1. At lower scan rates, the film formed was loosely adhered and

at higher scan rates there was poor polymer growth.

189

5.2.2.1 Voltammetric investigation

Cyclic voltammetry (CV)

The cyclic voltammogram for the electropolymerisation of PPyNQS is shown in Fig.

5.8a, and the potentiodynamic behaviour of the modified polypyrrole is at different

scan rates is shown in Fig. 5.8b. The electrochemically-polymerized film, grown at a

scan rate of 50 mVs-1 for 25 cycles, was observed to have good adhesion to the Pt

electrode surface. It should be noted that the ratio of the mole concentration of the

monomer/dopant mixture is critical in order to produce a conducting polymer. A

potential window of –400 through 700 mV vs. Ag/AgCl was used for the

polymerization and investigation after optimization of the synthesis conditions so as

to eliminate/minimize interference from other peaks. Additionally, this range is

required to prevent the over-oxidation and degradation of polypyrrole which normally

occurs at higher positive potentials, whilst a too negative potential will result in

hydrogen evolution [15].

(a)

190

(b)

Figure 5.8: Typical voltammogramms for (a) the polymerisation of Pt/PPYNQS

from 0.1 M Py + 0.01 M NQS in 0.05 M HCl. (25 cycles) at 50 mV/s and (b)

multi–scan rate voltammograms of the thin film in 0.05 M HCl at scan rates of 5,

10, 15, 20, 25, 30, 40 and 50 mV/s.

The cyclic voltammograms consistently displayed one distinctive oxidation and one

reduction peak. The peak separation between the anodic and cathodic peaks, increased

with increase in scan rates. From our multi–scan rate voltammograms (Fig. 5.8b), the

anodic/cathodic peak potentials at 5 mVs-1 were 364 mV and 301 mV vs. Ag/AgCl

respectively. The average formal potential (Eº′’ ) estimated from peak potentials at 5,

10, 15, 25, 30, 40 and 50 mVs-1 was calculated as 322 ± 5 mV. The peak separation of

63 mV obtained on the CV at 5 mVs-1 was used as an indication of a one electron

191

process for the polymer (n = 1). Applying equation 3.2 gives an n value of 1.1.

Similar electron transfer of unity had been reported for the electropolymerisation of

polypyrrole doped with p-toluenesulphonate, [16].

Test for reversibility

In order to access the degree of reversibility of the oxidation and reduction processes

of PPyNQS, the following diagnostic tests attributed to Nicholson [17] and Matsunda

[18] were performed:

a) The change in |Ip| vs. square root of scan rates (ν1/2)

b) The |Ip,a / Ip,c | value was determined provided (αa = αc = 0.5)

c) The shifts in Ep,c with increasing ν was monitored

d) The change in ∆Ep with increasing ν was monitored and compared

with (59/n) mV obtainable for reversible systems

Data from the multi scan rate voltammetry at 5, 10, 15, 20, 25, 30, 40 and 50 mVs-1

(Fig 5.8b) were used to perform these diagnostic tests. Figure 5.9a shows the variation

of anodic and cathodic peak current with square root of the scan rates, while Fig. 5.9b

shows the variation of the modulus of both the anodic and cathodic peak current with

square root of the scan rates. Figure 5.9b shows that |Ip| increases for both anodic and

cathodic peak currents with increases in ν1/2. However, the relationship is not

proportional as |Ip| / ν1/2 is not a constant in either case. Figure 5.9b further depicts the

best-fit line for all data points with a slope of 1.0309 x 10-3 A / (V/s) ½ (correlation

coefficient of 98.1%); and a slope of 0.8025 x 10-3 A / (V/s) ½ (correlation coefficient

of 99.1%) for the anodic and cathodic peaks, respectively. Thus the results obtained

for test (a), satisfy the condition for quasi-reversibility. Based on the assumption that

the anodic and cathodic transfer coefficients (αa and αc) are equal to 0.5 for simple

electron transfer [19], the diagnostic test (b) reveals that ratio of the modulus of the

two peak currents shown in the multi plot voltammogram (Fig. 5.8b) gives 1.30 ±

0.05 which is approximately 1 over the range of 5, 10, 15, 20, 25, 30, 40 and 50 mVs-

1. This is in line with the specifications of quasi-reversible systems. Test (c) which

investigates the shifts in Ep,c with increasing ν is shown in Fig. 5.10a. It is seen that as

the scan rate increases the cathodic peak potential shifts to lower values, thus,

providing further evidence for quasi-reversibility. The final test (d) which investigates

192

the behavior of the peak separation (∆Ep = Ep,a – Ep,c) with increases in scan rates is

shown in Fig. 5.10b. This indicates that the ∆Ep increases with increasing scan rates

because the cathodic peak potentials are becoming more negative while the anodic

peak potentials adopt more positive values.

It could therefore be generalized based on the compliance of the results with

Nicholson/Matsunda criteria that the Pt/PPyNQS system in the 0.05 M HCl

electrolyte undergoes a quasi-reversible electrochemistry under the conditions used

for the investigation.

(a)

193

(b)

Figure 5.9: (a) Plot of variation of anodic and cathodic peak currents with

square root of the scan rates and (b) plot showing the variation of the modulus of

both the anodic and cathodic peak currents with square root of scan rates of

Pt/PPyNQS in 0.05 M HCl.

194

(a)

195

(b)

Figure 5.10: (a) Plot showing the variation of cathodic peak potentials with scan

rates and (b) plot of peak separations versus scan rates of Pt/PPyNQS in 0.05 M

HCl at a 1.6 mm diameter Pt electrode at a temperature of 25 °C based on the

data from Fig. 5.8b.

Pulse techniques (DPV and OSWV)

The redox properties of the Pt/PPyNQS film were investigated using pulse techniques.

DPV investigation at a scan rate of 5 mVs-1 gave both anodic and cathodic peaks at

317 mV thus indicating an Eθ' estimate of 317 mV (Fig. 5.11). Also, the OSWV (Fig.

5.12) at a frequency of 15 Hz and amplitude of 25 mV gave one oxidation and one

reduction peak within the potential window of -400/700 mV. The average of the peaks

at 360 and 273 mV was used as an estimate for the formal potential (Eθ') value, being

317 mV.

196

Figure 5.11: Differential pulse voltammograms for the anodic and cathodic wave

difference for Pt/PPyNQS film in 0.05 M HCl using a scan rate of 5 mV/s and 50

mV pulse amplitude.

197

Figure 5.12: Square wave voltammogram for the anodic and cathodic wave

difference for Pt/PPyNQS film in 0.05 M HCl using frequency of 15 Hz and 25

mV amplitude.

5.2.2.2 Kinetic analysis of the Pt/PPyNQS system

The Brown Anson model was used to estimate the surface concentration of the

polymer (Γ*PPyNQS) using the peak currents (Ip) obtained at different scan rates (ν)

between 5 mVs-1 and 50 mVs-1 [20]. Thus from equation 3.5:

Ip = n2 F2Γ*PPyNQS Aν / 4RT

198

The notations F, A, R and T are constants for the Faraday’s constant (96584 C mol-1),

working electrode area, molar gas constant and room temperature of 298 K

respectively. The plots in Fig. 5.13 showed linear relationship with correlation

coefficient (r2) of 0.994 and 0.999 respectively for the cathodic and anodic currents

vs. scan rates. The ratio of anodic peak current (Ip,a) to cathodic peak current (Ip,c)

ranged from 1.23 to 1.38 over the range of scan rates applied, which is not exactly

unity as required for fully reversible one electron transfer, which further gave

credence to the quasi-reversible electron transfer mechanism that was suggested. The

plot confirms the formation of a stable film whose density is slightly lower during

reduction (1.42 x 10-7 mol.cm-2) as compared to oxidation (1.83 x 10-7 mol.cm-2). This

computation was based on the Brown Anson model using the cathodic slope of 2.687

x 10-3 A.s./V and the anodic slope of 3.478 x 10-3 A.s./V. The intercept values, for

non- faradaic currents caused by charging of the electrical double layer at the

electrode interface were close to zero being 8.50 x 10-6 A for the cathodic plot and

9.47 x 10-6 A for the anodic plotm (Fig. 5.13). This further lays credence to the quasi-

reversible electrochemistry of the system.

The systems displayed progressive shift in anodic peak potential towards more

positive values coupled with shift in cathodic peak potential to less positive values

with increase in scan rate. The peak separations increase progressively from 63 mV at

5 mVs-1 to 166 mV at 50 mVs-1 coupled with increase in the magnitude of the peak

currents with increase in scan rates. This shows that the peak currents are diffusion

controlled. Thus the Randel-Sevčik equation (equation 3.6) was applied to determine

the diffusion coefficient (De) for electron hopping along the polymer chain [21].

Ip / ν1/2 = 2.686 x 105 n3/2 A Γ*PPyNQS.De

1/2

Ip is the peak current in A, n is the number of electron transferred, ν is the scan rate in

V s-1, n is the no of electrons transferred, A is the surface area of the electrode in cm2,

Γ*PPyNQS is the surface concentration of the polymer film in mol cm-2, De is the rate

charges transportation in cm2.s-1 along the polymer chain. The slopes of the linear

plots shown in Fig. 5.9b are 1.0309 x 10-3 and 0.8025 x 10-3 A / (V/s) ½ for the

oxidation and reduction scan waves with a corresponding correlation coefficient of

0.981 and 0.991 respectively.

199

Figure 5.13: Plot of the peak current dependence on scan rate for Pt/PPyNQS

prepared from 0.1 M Py + 0.01 M NQS in 0.05 M HCl and characterized in 0.05

M HCl.

The rate of charge transportation, De, along the polymer chain was found to be 1.02 x

10-6 cm2.s-1, being the same for the oxidation and reduction reactions. This suggests

that the deviation from full reversibility does not necessarily involve permanent

electronic changes to the bulk polypyrrole film upon potential cycling, but that some

other phenomenon is responsible for the kinetics observed. The cyclic

voltammogramm at a scan rate of 5 mVs-1 was used to investigate the rate constant

(kº) for electron transfer within the polymer chain using Nicholson treatment for a

quasi-reversible electrochemical system [17, 19-21]. Thus using equation 3.9, the kº

was estimated.

kº = φ (α. n. F. ν. De / RT) ½

The transfer coefficient,α, of 0.5 was assumed for the PPyNQS systems and the

kinetic parameter, φ (dimensionless), was assigned a value of 7 based on the peak

200

separation, ΔEp, of 63 mV at a scan rate of 5 mVs-1 which indicated n = 1.1. The kº

value of 2.20 x 10-3 cm.s-1 obtained for the polymer at 5 mVs-1 shows that electron

hopping along the polymer chain at the low scan rate is quite facile. This is

comparable with the rate constants reported for other conducting polymers Vis. Pt/

polyaniline electrodes with kº values of 0.049 to 5.4 x 10-3 cms-1 in different

electrolytes [22] and Pt/poly(3,4-ethylenedioxythiophene) electrodes with kº values of

1.5 to 45.3 x 10-3 cms-1 when prepared under different conditions [23] under different

synthesis conditions.

5.2.2.3 Impedance spectroscopic investigation of Pt/PPyNQS

Electrochemical impedance spectroscopy (EIS) enables the separation of interfacial

electron transfer from concomitant electronic changes occurring within the bulk

material of the electrode on the basis of frequency dependent electrochemical

response. EIS data was collected, in consecutive 50 mV steps, in the potential range

corresponding to the oxidation of the polymer film (0 mV to 600 mV) and the

subsequent reduction by inversion of the steps in potential back to 0 mV. The values

obtained from the fit results of the data obtained from the impedance experiment

using the equivalent circuit in Fig. 5.14 is presented in tables 5.3 and 5.4. This

corresponds to the oxidative and reductive potential steppings respectively.

Typical data obtained at an applied potential of +50 mV vs. Ag/AgCl are shown in the

complex plane impedance plots of Fig.5.14 for a frequency range between 40,000 –

223 Hz. The data obtained over an extended frequency range of 100 KHz - 100 mHz

was analyzed using an equivalent electrical circuit consisting of the solution

resistance (Rs), an R1CPE1 parallel combination, where CPE is a constant phase

element, to model movement by electron hopping through the polymer film along the

polymer backbone, and a second R2C2 component in series representing the electrode

/solution interface, as indicated in [24, 25]. The CPE was modelled as a non-ideal

capacitance, according to equation 3.24

CPE = 1 / (Ciω) n equation 3.24

201

The CPE is defined by two values i.e. the capacitance, C, and the CPE exponent, n,

which has a value between 0.5 and 1 for a non-ideal capacitor. If n equals 1, the

equation is identical to that of a capacitor and smaller values can be related to surface

roughness and inhomogeneities, which lead to frequency dispersion. When a CPE is

placed in parallel to a resistor, a depressed semi-circle (Cole-Element) is produced.

Figure 5.14: Complex plane impedance plots of PPyNQS thin film electrode at

50 mV vs. Ag/AgCl in 0.05 M HCl during (•) step-by-step oxidation and (o)

subsequent reduction, (insert is the equivalent circuit used to fit the data).

202

Table 5.3: Analysis of the oxidative impedance data at the conductive polymer

electrode.

Potential

/mV

Rs

/Ωcm2

CPE1-T

/mFcm-2

CPE1-P Rct

/ Ωcm2

Cdl

/ µFcm-2

Rit

/ Ωcm2

50 10.54 0.98 0.597 20.88 28.0 4.22

100 10.11 1.98 0.553 17.76 28.8 3.74

150 10.00 2.58 0.538 14.81 32.9 3.13

200 9.67 2.94 0.543 11.87 32.9 3.14

250 9.59 3.40 0.532 11.19 34.7 2.87

300 9.68 3.55 0.529 10.76 35.5 2.77

350 9.85 3.14 0.545 10.88 34.7 2.74

400 9.55 3.30 0.533 11.52 34.5 2.95

Table 5.4: Analysis of reductive impedance data at the conductive polymer

electrode.

Potential

/mV

Rs

/ Ωcm2

CPE1-T

/mFcm-2

CPE1-P Rct

/ Ωcm2

Cdl

/ µFcm-2

Rit

/ Ωcm2

400 10.19 1.29 0.58 16.68 30.7 4.09

350 10.22 0.48 0.58 16.2 31.3 3.76

300 10.23 1.35 0.59 14.43 34.8 3.33

250 10.11 1.78 0.57 14.35 39.6 3.17

200 10.07 2.02 0.56 14.55 42.4 3.12

150 10.12 1.60 0.58 13.96 46.9 2.94

100 10.22 1.23 0.60 14.32 47.0 2.91

50 10.37 0.84 0.63 16.03 49.5 2.78

0 10.53 0.54 0.66 19.19 52.7 2.86

203

The solution resistance varied from 9.6 to 11.0 Ω. The values of R1 and CPE1 were

interpreted through the electrical characteristics of the electropolymerised PPyNQS

polymer bulk material. The capacitive nature of the bulk material was substantiated

by the inverse relationship between the capacitance and resistance values of the high

frequency loop. The polymer material showed good conductivity (10 – 20 Ω) and in

the region of the formal potential as obtained by voltammetric experiments, the

conductivity of the polymer material was measured as 10.8 Ω. The average value of n

for all experiments was 0.56, which is indicative of a rather rough morphology and

porous structure and is probably also associated with electron hopping along the

polymer backbone. The interface of the polymer with the HCl electrolyte showed

consistently low values of capacitance, C2 (30 μF) and resistance, R2 (2.7 - 4.2 Ω)

during the oxidation steps, the latter reaching a minimum value at the formal

oxidation potential. The electron transfer from solution to the polymer film during

oxidation is thought to be direct electron transfer without mediation by any surface

bound species, since the interfacial capacitance values of 16 μF hardly varied over the

potential range studied.

During the subsequent reduction steps, from 600 mV to 0 mV vs. Ag/AgCl, the

capacitance (2 mF) and the resistance (15.5 Ω) of the polymer bulk material remained

fairly constant. The bulk capacitance (CPE1) and resistance (R1) values were of the

same order of magnitude during the reduction steps as compared with oxidation.

However, a plot of the interfacial capacitance (C2) versus applied potential showed a

gradual increase in capacitance as the potential became more negative, and gradual

increase as it became more positive (Fig. 5.15). The capacitance values for the

oxidation and reduction trend intersect in the region of the formal potential (300 mV).

204

Figure 5.15: Plot of interfacial capacitance versus potential for the oxidation and

reduction of the PPyNQS thin film.

This trend suggests a disruption to the direct electron transfer kinetics observed during

oxidation. Evidence from SNIFTIRS equally suggests degradation of surface bound

polymer units by anions in solution (e.g. Cl-) which results in ring opening of surface

polymer units and eventual loss of CO2, similar to the structural degradation induced

in pyrrole under strongly basic conditions [26]. This surface chemistry could explain

the observed changes in EIS data and confirms the quasi-reversible oxidation kinetics

predicted by the scan-rate-dependent CV. The difference between the complex plane

impedance plots at 50 mV before oxidation and after reduction (Fig.5.15) is further

evidence of this.

Polymer structure and proposed mechanism for the surface chemistry

During the first cycle of electrochemical polymerization of pyrrole an inner layer is

formed upon which the polymerized polypyrrole grows. Subsequent cycling steps

allow for the development of the polymer chain resulting in the main layer and the

thickness thereof depends on the number of potential cycles employed. It is also the

main layer that influences overall film stability. The outer layer is produced by

polymer termination when the potential is turned off and is made up of mainly short

205

chain polypyrrole and therefore less stable than the main layer (Fig. 5.16a). When the

polypyrrole film is oxidized at 315 mV (vs Ag/AgCl) the surfactant anions (A-) align

close to the polymer surface to compensate for the positive charge caused by the

formation of bipolarons under anodic potentials (Fig. 5.16b). This phenomenon is

sometimes referred to as electro-neutrality coupling and results in electron hopping

within the polymer main layer to accommodate the bipolaron/ anion coupling at the

surface [27].

(a) Neutral polymer (b) Oxidized state

Figure 5.16. Model illustrating the alignment of charges at different oxidation

states of polypyrrole: (a) neutral polymer and (b) oxidized polymer.

While actuating the polymer in monomer free 0.05 M HCl, the entrapped 1, 2-

napthaquinone-4-sulphonate (NQS-1) anion is immobile, it does not move and the

hydrated small proton strictly determines ionic transport. However, with prolonged

actuation or overoxidation, there is breakdown of the surface bound pyrrole and

sulphonated units. The surfactant anion, NQS-1, which is formed upon dissociation in

the electrolyte behaves as a weak base. The surface bound pyrrole units loose the

protons on the nitrogen in order for the surfactant to return to the preferred acidic

state. The loss of the proton on the pyrrole rings however leads to disruption of the

cyclic structure by rearrangement to form hydroxyl and carbonyl species and eventual

loss of material through formation of CO2 [28].

206

This suggests that some pyrrole units on the surface of modified polypyrrole chain

undergoes ring opening [26]. The lone pair on the ring nitrogen combined with the

four π electrons of the two double bonds to give pyrrole an aromatic sextet of

electrons. The nitrogen is sp2-hybridised and acquires a positive charge as its lone

pair is delocalized around the ring [29, 30]. Therefore we propose a mechanism for

the oxidation of the polymer, which involves adsorption of the surfactant onto the

polypyrrole film on the electrode surface. The interaction of the surfactant anion with

the strongly polarized surface pyrrole units on the polymer backbone leads to

disruption of the induced aromaticity on the pyrrole. This results in the protonation of

the surfactant to yield the sulphonic acid and deprotonation of the ring nitrogen.

Further oxidation in the acid medium favors the reduction of the double bond adjacent

to the nitrogen, which leads to ring opening and subsequent rearrangement to produce

the more stable imine structure. The oxidation is irreversible and subsequent

electronic rearrangement transforms slowly to yield the capacitive material observed

under reduction conditions at around 200 mV vs. Ag/AgCl.

5.3 Modeling of the electrochemical and impedimetric properties of PPyNQS

In section 5.5.2.1, we established that the electrochemical behaviour of

electropolymerised PPyNQS on a platinum electrode involves a one electron, quasi-

reversible electrochemistry (ΔEp~60 mV), Eº′ of 322 mV and Ip,a / Ip,c is 1.3 with a

facile charge transport coefficient of 1.02 x 10-6 cm2s-1 [3]. Interfacial behaviour was

described with the equivalent circuit in Fig. 5.17 based on the characteristic

impedance profile obtainable at the potential (322 mV) where the polymer exhibits

maximum electro-activity (Fig. 5.18). The circuit defines two simultaneous kinetics;

one involving the bulk polymer material and the other taking place at the

electrolyte/polymer interface. The observed kinetics were investigated and used to

fashion out the relationship to changes in the analyte concentrations being sensed by

the transducer. The first intercept on the x-axis (Fig 5.18) is, typically, due to the

solution resistance Rs. It is independent from the applied potential for a given cell

configuration and typically about 10 Ohms. The high frequency semicircle is not fully

formed and hardly visible when the full frequency sweep is completed. However, by

207

expansion of the impedance data over a smaller range at the high frequency, it is

obvious that it do exist even though it was not fully formed.

Figure 5.17: Equivalent electrical circuit describing the electrical components of

Pt/PPyNQS in an electrolyte medium of 0.05 M HCl.

208

Figure 5.18: Typical Nyquist plot (top), Bode plot (bottom) for the Pt/PPyNQS

system.

209

It has already been reported that the series combination of a resistance, R, and a

constant phase element as in Fig. 5.17 leads to a depressed arc in the Y plane [31].

The impedance transformation from high to low frequency is highlighted in the Bode

plot (Fig. 5.18, bottom). The inability of the polymer to exhibit constant impedance

over some frequency range at the low frequency end is a major peculiarity of this

system.

The interrogation of the impedance profiles revealed that the least value of interfacial

real impedance at the lowest frequency (10 mHz) was obtainable at 322 mV (Fig.

5.19), which is the potential at which the most conductive state is obtainable. This

potential coincidentally gave the highest capacitance for the polymer. The interfacial

impedance values as we switch to lower potentials from 322 mV are progressively

higher because of the transformation from conducting polypyrrole state to the neutral

state. At higher potentials to 322 mV, gradual over-oxidation of polypyrrole sets in

which is accompanied with increase in interfacial impedance. Similar transformation

in interfacial impedance has been reported for polyaniline as it switches through the

emaraldine (conducting) and the pernigraniline and leuco-emeraldine (insulating)

states [32].

Figure 5.19: Plot of the interfacial impedance and capacitance at different

perturbation potentials of the Pt/PPyNQS electrode.

210

5.4 Test application of Pt/PPyNQS for benzidine and naphthalene

Pt/PPyNQS impedimetric sensor for benzidine and naphthalene The Pt/PPyNQS sensor was constructed using the electrochemically prepared film of

PPyNQS via potentiodynamic polymerization as described earlier. At the pre-fixed

potential of 322 mV, the impedance response generated by addition of the benzidine

analyte to the cell shows that the reactivity proceeds at the interface, followed with

that within the bulk polymer chain (Fig 5.20). The waveform of the plots in Fig. 5.20

shows that changes associated with the plot of the interfacial impedance () leads that

for the plot of the bulk polymer impedance ().

[Benzidene] µM0 100 200 300 400

Δ Im

peda

nce

(Ohm

s.cm

2 )

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

Bulk polymer Interface

N.B: Interfacial response leads that at the bulk polymer

Figure 5.20: Plot of changes in real impedance with increasing concentrations of

benzidine at the bulk polymers region () and at the interface ().

211

As the benzidine concentration was added to the electrolyte from 0 to 333 μM, the

impedance observed for the bulk polymer at the high frequency region (ca 10 KHz)

had values of 172 – 234 Ohms. The high frequency value of 10 KHz was chosen

because reproducible impedance measurements free of drift were obtainable as

against that obtained at the start up frequency of 100 KHz. At the interface mapped by

the low frequency region (ca 10 mHz), the impedance values were 840 -920 Ohms.

The relative change in the real impedance values at the two regions translates to 62

and 80 Ohms respectively. There is greater change in the interfacial impedance

compared to that of the bulk polymer impedance. In the contrary, we observed from

figures 5.21 and 5.22 that a more significant change in the capacitance values was

recorded for this system at the 0 and 333 μM extremes. While the bulk polymer

capacitance decreased from 72.7 to 57.1 μF, the interfacial capacitance decreased

from 4.7 to 3.7 mF. This translates to a difference in absolute capacitance values by a

factor of about 1000 times, with that at the interface being higher compared to that at

the bulk polymer (Fig. 5.21 and 5.22). This offers us a comparative advantage to

model the reactivity based on capacitance changes rather than the changes in the real

impedance.

Increasing concentration of BZD is insulating and thus increases the resistance of

the system. It also decreases the double layer capacitance at the electrode

interface. It could therefore be hypothesized that the sensing effect is occasioned

by the displacement of the double layer capacitance by the addition of the organic

compound. The interaction of the analyte (polar or non-polar) with the delocalized

positive charges along the polypyrrole chains induces changes in capacitance of

this nanostructured material.

The Pt/PPyNQS sensor effect is based on the normalized real impedance (Sn),

and/or that of the normalized real capacitance (Kn) as shown in equation 3.28 and

3.29 as used earlier by Hagen et al and described in chapter 3 [33].

Sn = |Zt| - |Z(t = 0)| equation 3.28

|Z(t = 0)|

212

Kn = |Ct| - |C(t = 0)| equation 3.29

|C(t = 0)|

When the Kn values (deviations of the magnitude of capacitance at a time t

containing a known concentration of the analyte from a value without analyte at t

= 0) for the electrochemical set up were plotted against increasing concentrations

of analytes, a hyperbolic curve defined by the equation: y = ax / (1 + bx) was

followed. Fig. 5.21 shows the plot for interfacial capacitance vs. [benzidine] with

a correlation coefficient, r2 value of 0.985 (n = 13), exhibiting a typical shape for

Michaelis-Menten kinetics. This could be used to determine benzidine

concentrations of 0 to 400 µM without total de-naturation of the sensor. A linear

plot defined by the equation y = 0.059x + 1.514 was observed at low

concentration range of 0 – 100 µM with a correlation coefficient, r2 of 0.992, (n =

5). From the linear plot (insert) shown in Fig. 5.21, low concentration range of 0 –

100 µM for benzidine could be determined with a sensitivity of 0.059 x 10-4

F/µM, i.e. 5.9 µF/µM, and detection limit of 3.407 µM. This concentration range

of 0 to100 µM for benzidine is obtainable in waste waters from contaminated

environment.

When similar plots were made for the normalized capacitance vs. benzidine

concentration at the bulk polymer region (Fig. 5.22), a seeming linear relationship

with r2 of 0.96 was seen as against hyperbolic trend seen for interfacial normalized

capacitance plots (Fig.5.21). This confirms that different kinetics of electron

transfer is taking place within the bulk polymer as well as at the interface.

213

[Benzidine] µM0 100 200 300 400

Δ C

apac

itanc

e (1

0-4 )

F

-2

0

2

4

6

8

10

12Interfacer2 = 0.985

0 20 40 60 80 10012345678

r ² = 0.992


benzidine at the interface with insert showing the calibration curve for the linear

region.

214

[Benzidene] µM0 100 200 300 400

Δ C

apac

itanc

e µ

F

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

Bulk polymerRegression line

b[0] = 2.7355304388e-3b[1] = 5.1260136441e-4r ² = 0.9603622854


benzidine at the bulk polymers region in () and the straight line showing the

regression line.

The kinetics involved in the interfacial impedance with benzidine concentration is

presented in Fig. 5.23. The normalized impedance increased gradually with small

concentration of benzidine after which it decreased in a sinusoidal pattern. This

transformation could be due to reaction between the polar groups of the analyte with

ionic species in the electrolyte, thus altering the conductivity. A direct relationship

between the impedance measurements and benzidine concentration could therefore

not be made because of the coupled reactions involved.

215

[Benzidene] µM0 100 200 300 400

Δ Im

peda

nce

(Ohm

s.cm

2 )

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

Interface

Figure 5.23: Plot of changes in impedance with increasing concentrations of

benzidine at the bulk polymers region in ()

Pt/PPyNQS impedimetric sensor for naphthalene in comparison to that for

benzidine.

In the investigation of the sensor response to naphthalene, the magnitude of the real

impedance was found to increase with increasing concentration of the analyte while

the double layer capacitance decays hyperbolically (Fig 5.24). This is expected

because the organic moiety naphthalene, is relatively insulating compared to the

electrolyte medium and thus should create higher impedance. The relative change in

capacitance from the naphthalene addition over the concentration range investigated is

smaller compared to that from benzidine. This could be attributed to the greater

electrostatic interaction at the polymer’s interface resulting from the polar benzidine

compared to the non-polar naphthalene. The hyperbolic decay for the naphthalene’s

capacitance plots (Fig 5.24) gave a much lower r2 value of 0.886 compared with that

from benzidine capacitance plots with r2 value of 0.992 (Fig 5.21). However, within a

216

narrow concentration range of 0 – 3 µM, a linear plot defined by y = 0.294x + 0.042

is satisfied with r2 = 0.980 (n = 3) for the naphthalene concentration vs. capacitance

plot (insert of Fig 5.24). This offers room for the development of capacitance-based

sensor for the determination of benzidine and naphthalene at ppm and ppb levels.

The usage of the sensor for high concentration of benzidine solutions has a fouling

effect on the sensor. At concentration of 340 µM and above, there was gradual

deterioration of the real impedance trend and loss of interfacial electrochemistry (Fig

5.21 and 5.23). The naphthalene based capacitance sensor showed similar saturation

at naphthalene concentrations in excess of 150 µM. The naphthalene solution brought

about the saturation of the sensor at a lower concentration because of its higher

insulating capability compared with that of benzidine.

[Naphthalene] µM0 100 200 300 400

Δ C

apac

itanc

e (1

0-4 )

F

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.0

0.2

0.4

0.6

0.8

1.0

r ² = 0.984

r2 = 0.886


naphthalene at the interface with insert showing calibration curve for the linear

region.

217

The electroactivity of the polymer is hindered by the repulsion to the ingress/egress of

counter ion at the polymers interface due to the restriction caused by the large NQS

moiety. This factor is responsible for the decrease in the double layer capacitance with

increase in analyte concentration. Higher concentration range of these pollutants

could be determined by using the hyperbolic curve of the normalized capacitance vs.

analyte concentrations. Polycyclic aromatic hydrocarbon (naphthalene) showed a

higher propensity to displace the interfacial double layer capacitance than the polar

benzidine moiety.

Conclusions

1, 2-napthaquinone–4-sulphonate have been successfully incorporated into the matrix

of polypyrrole on a platinum disc electrode through potential cycling in aqueous

solution of pyrrole-monomer (0.1 M) and the sodium salt of 1, 2-napthaquinone–4-

sulphonic acid-dopant (0.01 M) in 0.05 M HCl. The resulting polymer showed high

conductivity, optical changes, spectroscopic transitions and good electroactivity. The

polypyrrole thin film microelectrode was characterized electrochemically for the first

time and found to exhibit good electronic and spectroscopic properties. Voltammetric

investigations showed that the polymer exhibited quasi-reversible kinetics in a

potential window of -400 mV to 700 mV, with a formal potential of 322 mV vs.

Ag/AgCl. The diffusion coefficient was calculated to be 1.02 x 10-6 cm2 s-1 for a thin

film with a surface concentration of 1.83 x 10-7 mol cm-2 having a rate constant of

2.20 x 10-3 cm.s-1 at 5 mVs-1. The self-assembled film of the nanostructured-modified

polypyrrole showed good adherence on platinum disc electrode (Pt/PPyNQS) and

showed potential for use as anode for environmental sensor applications. The

modified polypyrrole is composed of nano micelles with diameters of 50 – 100 nm. It

displays high electroactivity and quasi-reversible electrochemistry. It also shows

stronger electrochemical activity than conventional PPy, improved solubility and

strong UV-Vis absorptions at 380, 750 and 820 nm for the polaron, bipolaron and

over lapped bi-polaron bands respectively. There was good correlation of data

between the electrochemical and spectro electrochemical investigation on the

modified polymer using cyclic voltammetry, differential pulse voltammetry, square

wave voltammetry, electrochemical impedance spectroscopy and SNIFTIRS. During

SNIFTIRS investigation of Pt/PPyNQS thin film, there was smooth transition from

the neutral to the polaronic and bipolaronic states as it was switched through a

218

potential window of –400 through 700 mV vs. Ag/AgCl. It is worth noting that the

NQSA doped polypyrrole showed strong emissions in aqueous dispersion, which

provides indication of potential applications in fabrication of various optometric

devices. The use of the Pt/PPyNQS novel impedimetric smart sensor for the detection

and quantification of benzidine and naphthaline in aqueous solutions proves

promising based on our preliminary investigation. Our test application of the

Pt/PPyNQS shows that sensors with good linear relationship (r2 > 0.98) between

capacitance and concentration for benzidine and naphthalene could be constructed

using a concentration range of 0 – 100 µM and 0 – 3 µM for benzidine and

naphthalene respectively.

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Chapter Six

Results and discussion 3

Spectroscopic and morphological studies of polypyrrole composites

with metal oxides (PPyWO3 and PPyZrO2)

This chapter presents results and discussion on the chemically synthesised metal

oxide modified polypyrrole. The hybrid materials based on WO3-x and ZrO2-y and

polypyrrole were prepared from the in-situ oxidation of pyrrole in acidic solution of

the individual metal oxide. Results obtained from the morphological and

spectroscopic investigation are discussed.

6.0 Introduction

Metal oxide modified polypyrrole have been used as organic/inorganic hybrid

electrodes for increased charge storage in electrochemical storage devices such as

supercapacitors and batteries [1-3]. While transition metal oxides incorporated into

the matrix of double layer activated carbons are used as double layer capacitors, based

on their high specific capacitance and relatively low electric conductivity, the metal

oxide nanohybrid-polypyrrole offers room for increased charge storage capacity and

conductivity because of the advantage of being both electroactive and conductive. The

effective energy storage generated from the nanohybrid of a fairly conducting

transition metal oxide with an electroactive and conducting organic polymer like

polypyrrole offers room for improved technological possibilities such as

electrochemical sensors and supercapacitors. In this study, in-situ reaction between

pyrrole and tungsten (VI) oxide or zirconium (IV) oxide in acidic solutions are used to

produce insoluble polypyrrole composites via oxidation with aqueous solution of

ammonium persulfate. While the pyrrole is oxidized to polypyrrole, the metal oxide in

solution also gets reduced to form insoluble particles in the process. The simultaneous

reactions coupled with stirring allows for the incorporation of the insoluble metal

oxide particles into the interstitial pores of the polymer. The polymer yield,

224

morphology and spectroscopic properties of the metal oxide polypyrroles are

discussed underneath.

6.1 Chemical synthesis of (PPyWO3 and PPyZrO2)

The dry product of the tungsten oxide modified polypyrrole (PPyWO3) and zirconium

oxide modified polypyrrole (PPyZrO2) obtained using the experimental procedure

described in Table 3.3 and 3.4 was investigated for their morphological and

spectroscopic properties.

6.1.1 Yield pattern of metal oxide modified polypyrroles

The yield obtained from the chemical synthesis in acidic solution prepared from a

metal-oxide:pyrrole mole concentration ratio of 0.8 and oxidant:pyrrole mole

concentration ratio of 0.2 and 1.0 for PPyWO3 and PPyZrO2 is presented in Table 6.1.

The pH indicated was determined after the polymerization reaction in the 0.1 M HCl

(pH = 1.2) is completed. The % yields represent the % of the dry polymer (g)

obtained from the pyrrole used (ml). The polymerization yields of the hybrid

polymers prepared from the higher o/m mole concentration ratio is higher than those

from the lower o/m mole concentration ratio at a fixed d/m mole concentration ratio

of 0.8. More polymer hybrid is obtained from the tungsten oxide modified

polypyrroles than that from zirconium oxide. This might be due to the relatively

higher mole equivalent of the tungsten oxide leading to higher weight for the

precipitated polymer. The evidence for the incorporation of the metal oxides into the

polymer matrix was provided by stigmation of the polymers micrograph with energy

dispersive x-ray (EDX) within the scanning electron microscope chamber. The EDX

spectra provided information on the proximate elemental analysis for the transition

metal, carbon, oxygen and sulphur in the nanoparticles assayed. Optimisation of the

chemical synthesis conditions to determine desirable maximum loading of metal

oxide into the polymers matrix for the production of a highly conducting polymer

composite state is still being investigated.

225

Table 6.1: Yield of metal-oxide modified polypyrroles prepared under different

synthesis conditions.

Code pyrrole

used

(ml)-m

metal-

oxide

used (g)-d

APS

used (g)

-o

ratios

[d/m and

o/m]

pH

colour

change

polymer

yield

(g/ml)

PPyWO3-

A

0.529

mL

(0.0075

mol)

1.391 g

(0.0060

mol)

0.3484 g

(0.0015

mol)

d/m (0.80)

o/m (0.20)

1.68 Light green to

black after

oxidation

1.138 g

(210 %)

PPyWO3-

B

0.529

mL

(0.0075

mol)

1.391 g

(0.0060

mol)

1.745 g

(0.0075

mol)

d/m (0.80)

o/m (1.00)

1.36 Light green to

black after

oxidation

1.889 g

(357 %)

PPyZrO2-

A

0.529

mL

(0.0075

mol)

0.740 g

(0.0060

mol)

0.3484 g

(0.0015

mol)

d/m (0.80)

o/m (0.20)

1.77 White milky

solution to

black

0.753 g

(142 %)

PPyZrO2-

B

0.529

mL

(0.0075

mol)

0.740 g

(0.0060

mol)

1.745 g

(0.0075

mol)

d/m (0.80)

o/m (1.00)

1.37 White milky

solution to

black

1.215

(230 %)

* defined as follows:‘d/m’ is mole concentration ratios of metal oxide to monomer

(pyrrole); ‘o/m’ is mole concentration ratios of oxidant (APS) to monomer (pyrrole)

6.1.2 Morphological and EDX examination

The SEM micrograph on the polypyrrole from the o/m mole concentration ratio of 0.2

was taken to see if nanohybrids were formed and to determine relative metal oxide

loading in the polymer matrix. Figures 6.1 show the fibrillar morphology of the

nanofibres forms of the polypyrrole hybrids. The micrograph of the dry, granular

powder of the tungsten oxide modified polypyrrole (Fig. 6.1a) gave agglomerated

226

nanobundles with circular diameters of about 75 - 300 nm while that from zirconium

oxide modified polypyrrole (Fig. 6.1b) gave islands of globular nanobundles of

similar diameter as PPyWO3. Similar morphology with spherical particles of particle

sizes of 400-500 nm was reported for nanohybrid of PPy/Fe2O3 nanocomposite [3].

(a)

227

(b)

Figure 6.1: SEM micrographs of dry powder of metal oxide modified polypyrrole

showing the typical fibrous nanostructures from (a) PPyWO3 [d/m 0.8; o/m 0.2]

and (b) PPyZrO2 [d/m 0.8; o/m 0.2].

Table 6.3 presents the proximate elemental composition obtained from the EDX

spectra of different polypyrroles. Evidence of the incorporation of the metal oxides

was revealed in the respective metal oxide modified polypyrrole. The higher product

yield observed from the PPyWO3 was also corroborated by the higher metal loading

of about 12 % for PPyWO3 as against about 8 % seen in PPyZrO2 (Table 6.3).

Furthermore, the data shows lower % carbon from the metal oxide modified polymers

(PPyWO3 and PPyZrO2) compared to the organic acid modified polymers (PPyNSA

and PPyNQS). The order of decreasing % carbon is PPyNSA > PPyNQS > PPyZrO2

> PPyWO3. The relatively lower % sulphur in the metal oxide modified polypyrrole is

attributed to the absence of surfactants whereas the sulfonated polypyrroles have

higher % sulphur. The observed 1.5 % sulphur in the metal oxide modified

228

polypyrrole must have originated from the oxidant (APS) used in the polymers

preparation. Higher values for the surfactant modified PPy is therefore not surprising.

Table 6.2: Comparative trend of elemental composition (C, S, O, W, Zr, others)

in different modified polypyrroles prepared at from d/m ratio of 0.8 and o/m

ratio 0.2 by EDX spectroscopic analysis.

samples C (%)

S (%)

O (%)

W (%)

Zr (%)

others (%)

total (%)

PPyNSA 91.84 5.24 2.66 - - 0.26 100.00

PPyNQS 89.37 5.46 4.58 - - 0.67 100.08

PPyWO3 82.55 1.47 4.05 11.77 - 0.22 100.06

PPyZrO2 87.23 1.15 3.92 - 7.55 0.15 100.00

6.2 Spectroscopic properties of metal oxide modified polypyrroles

6.2.1 UV-Vis Spectroscopy

Figures 6.2 present the UV-Vis spectra for the metal oxide modified polypyrrole.

Similar trend of spectra were obtained on the polymer composites of PPyWO3 (Fig.

6.2A) and PPyZrO2 (Fig. 6.2B) using different o/m mole concentration ratios of 0.2

and 1. For the PPyWO3, a small absorption maxima at 325 nm for the conjugated

double bonds (i.e. π to π* transition) is indicated. The red shift from the normal π to

π*absorption at about 295 nm might be due to the inclusion of the solvated tungsten

oxide along the polymers matrix [4]. The polaronic absorption is poorly formed at

about 480 nm (Fig. 6.2A). Similar trend of spectra were seen for the PPyZrO2 (Fig.

6.2B). Absence of sharp polaronic absorption and bipolaronic absorption at higher

229

wavelengths is an indication of insignificant charge carriers in the system and further

optimisation may be required to improve the polymers conductivity.

(A)

230

(B)

Figure 6.2: UV-Vis results for metal oxide modified polypyrrole prepared under

different synthesis conditions: Fig. 6.2A: PPyWO3 from o/m 0.2 and 1.0; and Fig.

6.2B: PPyZrO2 from o/m 0.2 and 1.0.

6.2.2 FTIR spectral studies

Figure 6.3 shows the IR spectra of the metal oxide modified polypyrrole. Fig. 6.3A

gives spectra for PPyWO3 and metal oxide free polypyrrole (PPyDW) while Fig. 6.3B

gives the spectra for PPyZrO2 and metal oxide free polypyrrole (PPyDW)

respectively. The Tables 6.3 and 6.4 present’s data on some characteristic bands for

231

the polymers and compared with those reported for polypyrrole by Geetha and Trivedi

[5]. All the characteristic absorption bands for the metal oxide modified polypyrrole

were observed especially at the fingerprint region of 1000 – 1700 cm-1 with slight

variation in the absolute values (Tables 6.3 and 6.4). The usual N – H stretching at

3400 cm-1 in neutral polypyrrole is present in both PPyWO3 and PPyZrO2 samples,

though with slight variations, indicating that the polymers are not in the doped state as

was observed in the earlier investigation on PPyDW, PPyNSA (d/m 0.8, o/m 0.2) and

PPyNSA (d/m 0.8, o/m 1.0) [6].

The most interesting feature in the IR spectra is the lower wavelength absorption band

at 822 cm-1 for PPyWO3 (Fig. and 558 cm-1 for PPyZrO2. Similar low band absorption

at around 500 cm-1 was reported for polypyrrole-manganese (IV) oxide

nanocomposite hybrids [1]. Furthermore, an extra sharp peak due to the C=C/C-C

vibrational modes at 1636 – 1720 cm-1, depending on the metal ion and the oxidant

concentration used, was seen in the spectra of the metal oxide modified polypyrroles.

This peak is assigned to the shifted peak from 1540 cm-1 due to significant

overoxidation of the polypyrrole matrix [1, 6]. The oxidising nature of the metal oxide

coupled with that of the oxidant might have caused this over-oxidation. This will

invariably reduce the electronic conductivity and ultimately loss or reduced

electrochemical activity. These characters in the metal oxide modified polypyrrole

attest to the incorporation of the metal oxide in the polypyrrole. The included

inorganic component in the hybrid is a reduced metal ion resulting from the oxidative

polymerisation. Thus, the reduced for in which the metal oxide precipitate could be

represented as WO3-x and ZrO2-y respectively where x and y are integers showing the

extent of oxidation in each case [1].

232

(A)

233

(B)

Figure 6.3: FTIR spectra of metal oxide modified polypyrroles in KBr medium,

Fig. 6.3A: (a) PPyDW, (b) PPyWO3 (d/m 0.8; o/m 0.2) and (c) PPyWO3 (d/m 0.8;

o/m 1.0); and Fig. 6.3B: (a) PPyDW, (b) PPyZrO2 (d/m 0.8; o/m 0.2) and (c)

PPyZrO2 (d/m 0.8; o/m 1.0).

234

Table 6.3: Major shifts of bands (cm-1) in FTIR spectra of PPyDW, PPyWO3

(d/m 0.8, o/m 0.2) and PPyWO3 (d/m 0.8, o/m 1.0) from undoped polypyrrole


Major Bands PPyDW PPyWO3 PPyWO3

(cm-1) (o/m 0.2) (o/m 1.0)

3421 (N-H Str) Absent 3420 3413

3100 (C-H Str) Absent Absent Absent

1535 (C=C &

C-C Str) 1554 1550 1547

1450 (N-H Str) Absent 1461 1458

1295 (C-H &

N-H Def) 1308 1305 1305

1050 (C-H Def) 1047 1040 1040

235

Table 6.4: Major shifts of bands (cm-1) in FTIR spectra of PPyDW, PPyZrO2

(d/m 0.8, o/m 0.2) and PPyZrO2 (d/m 0.8, o/m 1.0) from undoped polypyrrole


Major Bands PPyDW PPyZrO2 PPyZrO2

(cm-1) (o/m 0.2) (o/m 1.0)

3421 (N-H str) Absent 3461 3468

3100 (C-H str) Absent Absent Absent

1535 (C=C &

C-C str) 1554 1556 1558

1450 (N-H str) Absent Absent Absent

1295 (C-H &

N-H def) 1308 1311 1307

1050 (C-H def) 1047 1045 1045

Conclusions

This preliminary investigation on metal oxide modified polypyrrole has shown that

significant modification of the polypyrrole matrix is generated by the in-situ

polymerisation. The metal oxide nanoparticles served as a support to the

polymerisation process of pyrrole and lead to a more porous structure with a higher

specific surface area. Nanohybrids are readily formed via the in-situ polymerisation in

acidic aqueous medium. Two simultaneous processes have been hypothesised; namely

the oxidation of pyrrole to polypyrrole and the reduction of the WO3 to WO3-x (or

ZrO2 to ZrO2-y). The investigated polymers do not show significant features indicative

of reasonable composition of charge carriers required for electrocatalytic application

and further optimisation of synthesis conditions to come up with an ideal metal oxide

loading is required. Evidence of incorporation of the metal oxide into the polymers

matrix was provided by both EDX and FTIR spectroscopic analysis. Electrochemical

236

characterisation and test application of the nanohybrids will be looked into in future

work. The likely scope of application of the hybrid polymers are in electrochromic

devices and metal oxide sensors (MOS) that are operated at elevated temperatures.

References

1. Cuentas-Gallegos, A.K. and Gómez-Romero, P. (2005). In-Situ Synthesis of

Polypyrrole-MnO2−x Nanocomposite Hybrids, J. New. Mat. Electrochem. Systems, 8,

181-188.

2. Rocco, A.M., De Paoli, M-A., Zanelli, A. & Mastragostino, M. (1996). An

electrochromic device combining polypyrrole and WO3 –I. Liquid electrolyte,

Electrochimica Acta, 41(18), 2805-2816.

3. Mallouki, M., Tran-Van, F. Sarrzin, C., Simon, P., Daffos, B. De, A, Chevrot, C.

and Fauvarque, J. (2007). Polypyrrole-Fe2O3 nanohybrid materials for

electrochemical storage, J. Solid State Electrochem, 11, 398-406.

4. Kemp, W. (1988). Organic Spectroscopy, ELBS/Macmillan, 2nd ed., 204 – 206.

5. Geetha, S. & Trivedi, D.C. (2004). Studies on polypyrrole film in room temperature

melt, Materials Chemistry and Physics, 88, 388–397.



491(1-2), 117-125.




237

Chapter Seven

Conclusions and Recommendations

7.1 Conclusions

This research work has investigated the aqueous synthesis, characterization and

amperometric/impedimetric applications of some polypyrrole involving modification

with different polyaromatic hydrocarbon sulphonic acid (PAHSA) and transition

metal oxides of WO3 and ZrO2. The PAHSA used are β–naphthalene sulphonic acid

(NSA) and the sodium salt of 1, 2-napthaquinone-4-sulphonic acid (NQS). In order to

follow the transition in the properties of the modified polypyrrole with different

synthesis conditions, controls synthesis were carried out as follows:

Polypyrrole was chemically synthesized using oxidant of ammonium

persulphate (APS) without the use of acidic medium of hydrochloric acid as

the electrolyte (PPyDW),

Polypyrrole was chemically synthesized using oxidant of ammonium

persulphate (APS) in acidic medium of hydrochloric acid as the electrolyte

(PPyHCl) otherwise referred to as polypyrrole (PPy).

The investigation involved synthesis at different temperatures, dopant to monomer

mole concentrations ratios (d/m), oxidant to monomer mole concentrations ratios

(o/m), use of different electrodes for electrochemical synthesis and variation of ph and

electrochemical potential window. A general electrochemical characterization of the

polymers was undertaking to ascertain electroactivity of the derived polymers.

Morphological and spectroscopic characterizations were also used to determine the

existence of nanostructures and their intrinsic properties. Test applications on the

modified polypyrroles with good morphological, spectroscopic and electrochemical

properties were investigated for some common pollutants in waste water. The testing

was aimed at determining if there is amperometric or impedimetric response to

changing concentrations of the pollutants in waste water. The study have provided a

platform for the production of various chemical and biological sensors based on the

238

broad evaluation of the morphology, spectroscopic, electrochemical and test

applications explored in this study.

7.2 Main scientific contributions of the dissertation

Production of various nanostructures of polypyrrole using surfactant mediated

synthesis was successfully carried out within the first one year of this study.

The polymerization of pyrrole in β–naphthalene sulphonic acid (NSA) gave

nanotubules, nanomicelles or nanosheets of polypyrrole (PPy) morphologies

depending on the amount of NSA in the polymer and the temperature of the

reaction. The modified polypyrrole from NQS is composed of mainly

nanomicelles with diameters of 50 – 100 nm.

Validation of the electrochemistry of the various polypyrrole transition states

during cycling. Electrochemical analysis of PPyNSA reveals two redox

couples: a/a′ - partly oxidized polypyrrole-naphthalene sulphonate radical

cation/neutral polypyrrole naphthalene sulphonate; b/b′ - fully oxidized

naphthalene sulphonate radical cation/partly reduced polypyrrole-naphthalene

sulphonate radical anion which are closely associated. However, CV, SWV

and EIS showed that PPyNQS had single redox couple with a quasi reversible

electrochemistry at low scan rates.

Modification of polypyrrole with NQS was carried out for the first time and

conditions for the chemical and electrochemical synthesis were optimized.

Validation of the electrochemistry of the nanostructured modified polypyrrole

(PPy) self assembled with sodium salt of 1, 2-napthaquinone-4-sulphonic acid

(NQSA) and pyrrole (Py) on platinum disc electrode (Pt/PPyNQS). The

incorporation of the NQS dopant in the polymer matrix was validated by in-

situ SNIFTIR investigation.

The synergy of synthesis conditions, morphology of modified polypyrroles,

UV-Vis and FTIR properties of the polymer materials in different solvents,

and the electrochemical properties at different electrodes and potentials were

used to establish optimum synthesis conditions for PPyHCl, PPyNSA,

PPyNQS. Further optimizations are still required for the metal oxide modified

polypyrroles.

Kinetic study shows PPyNQS had slightly higher rate constant (ko) than

PPyNSA being 3.08 x 10-2 cm s-1 @ 100 mV/s for PPyNSA and 3.75 x 10-2

239

cm s-1 @ 100 mVs-1 for PPyNQS which are close to literature values for

other conducting polymers.

Beside, the PPyNQS adhered better to the electrode surface than the PPyNSA

and it is relatively more stable to many potential cycling.

Impedimetric investigation attest to redox activities at potential between 200 -

300 mV for PPyNSA and 320 mV for PPyNQS at the polymers/electrolyte

interface.

Test application of the GCE/PPyNSA electrode as phenol sensor in aqueous

acidic medium showed that PPyNSA based sensor can be used in the

amperometric detection of phenol in water and environmental samples. The

dynamic linear range of the sensor showed that the sensor is analytically

useful at phenol concentrations of 0.65-139.5 µM. This translates to phenol

mass concentration of 6.7-1500 parts per billion (ppb). This is within the range

of phenol found in pharmaceutical industries and refinery effluents.

Test application of the Pt/PPyNQS electrode as benzidine sensor in aqueous

acidic medium showed that PPyNQS based sensor can be used in the

impedimetric detection of phenol in water and industrial waste water effluents.

An impedimetric transduction mechanism was proposed for the interaction of

analytes of benzidine and naphthalene with a film of PPyNQS on platinum

disc electrode based on the high impedimetric pattern observed at low

frequency.

7.3 Recommendations for future work The polyaromatic hydrocarbon sulphonic acid (PAHSA) and transition metal oxides

evaluated in this dissertation represent only a sample of their respective groups. While

more conclusive study is required on the metal oxide modified polypyrrole, further

evaluation on other PAHSA should be explored for their potential use as

electrocatalytic sensor devices. The developed sensor systems based on PPyNSA and

PPyNQS should be explored for use as biological sensors at neutral pHs.

The morphological study on the electrochemically generated films in this study could

not be carried out due to lack of instrumentation for in-situ simultaneous

240

electropolymerisation and morphological evaluation. It will be needful to further

validate that the features observed on the chemically synthesized polymers as being

readily reproducible or improved upon in electrochemically generated film.

The inability of the polypyrrole used in this study to attain a constant impedance value

at low frequency is a major limitation in the development of direct impedimetric and

capacitive sensor in this study. Conditions for the attainment of this desirable

impedance pattern for the polymer systems should be explored.

Obviously, the ultimate objective is to have assembly of chemical sensors for

determination of environmental pollutants, particularly the anthropogenic ones such

as benzenoids e.g. benzene, toluene, ethylbenzene and xylene isomers –BTEX, that

are also volatile organic pollutants -VOC’s; chlorinated compounds e.g.

trichloroethylene, -TCE; polycyclic aromatic hydrocarbons -PAHs, e.g. naphthalene,

fluroanthene, pyrene; and polychlorinated biphenyls (PCBs) and other priority organic

pollutants (POP’s) such as benzidine, phenol and its derivatives. The development of

E-Noses that can determine many of these analytes simultaneously is an objective that

should be met in the nearest future.

7.4 Output from the dissertation 7.4.1 Contributions at conferences 1. Akinyeye, R., Iwuoha, E. & Baker, P.G.L.: Interfacial Characterisation of

Modified Conductive Polymer Sensor Materials, 8th UNESCO School and

IUPAC Conference on Macromolecules and Materials Science, 4-9 June 2005,

Flic en Flac, Mauritus.

2. Akinyeye, R.: Chemical and Electrochemical Synthesis of Polypyrrole

Impedimetric Sensors, Presentation to the Sensor Research Group, University

of the Western Cape, Bellville, South Africa, 6th December, 2005.

3. Akinyeye, R., Gwagwa, X. Y., Iwuoha, E. I. & Baker, P.G.L.: Interfacial

Characterisation of Modified Conductive Polymer Sensor Materials, 4th

241

International Conference on Instrumental Methods of Analysis: Modern

Trends and Applications, 2-6 October 2005, Iraklion, Crete Greece, Book of

Abstracts, p191.

4. Baker, P.G.L., Akinyeye, R.O., Sekota, M., Ahmed, A., Khan, R., Brett,

C.M.A., & Iwuoha, E.: Electrochemical Characterisation of Polypyrrole

Modified with 1, 2-Naphthaquinone-4-sulphonic acid, 11th International

Conference on Electrochemistry, ESEAC 2006, Bordeaux, France, 11–15 June

2006.

5. Akinyeye, R.: Chemical and Electrochemical Synthesis of Polypyrrole

Impedimetric Sensors, Presentation to the Department of Chemistry,

University of the Western Cape, Bellville, South Africa, 18th October, 2006.

6. Akinyeye, R.O., Michira, M., Klink, M., Somerset, V., Sekota, M., Ahmed,

A-Al., Ignaszak, A., Baker P. & Iwuoha, E.: Electrokinetics and Sensor

Applications of Polypyrrole Modified with 1, 4-Naphthaquinone Sulphonic

acid (NQSA), The 9th International Symposium On Kinetics in Analytical

Chemistry, Marrakech, Morocco, November 2–4, 2006, Book of Abstracts,

p21.

7. Iwuoha, E., Baker, P., Michira, M., Akinyeye, R., Sekota, M., Ahmed,A-

Al., Alemu H. & Ngila, J.C., Kinetic Modelling and Drug Metabolism

Biosensor for Antiretroviral Drugs, The 9th International Symposium on

Kinetics in Analytical Chemistry, Marrakech, Morocco, November 2–4,

2006, Book of Abstracts p33.

8. Akinyeye, R.O., Michira, M., Klink, M., Somerset, V., Sekota, M.,

Ahmed, A-Al., Ignaszak, A., Baker P. & Iwuoha, E. Polypyrrole 1, 4-

Naphthaquinone Sulphonate (PPYNS) Smart Nanomaterial for Impedimetric

Sensors, South African Nanotechnology Initiative, Nanoafrica 2006

International Conference, University of Cape Town, Capetown, South

Africa, November, 26–29, 2006, Book of Abstracts, p6.

242

9. Baker, P.G.L., Akinyeye, R.O., Michira, I. Sekota, M., Ahmed, A-Al., &

Iwuoha, E.: Electrochemical and Sensor Dynamics of Nanostructured

Polypyrroles, South African Nanotechnology Initiative, Nanoafrica 2006

International Conference, University Of Cape Town, Capetown, South

Africa, November, 26–29, 2006, Book of Abstracts, p13.

10. Iwuoha, E., Baker, P., Michira, M., Akinyeye, R., Sekota, M., Ahmed, A- Al.:

Electrochemical Interrogation of Cytochrome P450 (2D6) Nanobiosensors.

South African Nanotechnology Initiative, Nanoafrica 2006 International

Conference, University of Cape Town, Capetown, South Africa, November,

26–29, 2006, Book of Abstracts, p33.

11. Akinyeye, R.O., Michira, M., Klink, M., Somerset, V., Baker, P. &

Iwuoha, E.: Electochemistry and Application of Nanostructured Conducting

Polypyrrole Modified with β-Naphthalene Sulphonic Acid (NSA) for the

Determination of some Priority Organic Pollutants (POPs) in Waste Water,

38th National Convention of the South African Chemical Institute, University

of Kwazulu Natal, Durban, South Africa, December 3–8, 2006.

12. Akinyeye, R.O., Ignaszak, A., Ahmed, A-Al., Baker, P. & and Iwuoha, E.:

Synthesis, Electro-Kinetics and Sensor Applications of Sulphonated

Polypyrrole For The Determination of Anthropogenic Organic Pollutants, 6th

EBASI International Conference at Ithemba Labs, Stallenbosch, South Africa,

23rd –26th, January, 2007.

13. Akinyeye, R.O., Michira, M., Klink, M., Somerset, V., Arotiba, O. A.,

Owino, J.O., Ignaszak, A. Ahmed, A.Al, Waryo, T.T., Baker, P.G.L. &

Iwuoha, E.I.: Impedimetric Applications of Nanostructured Conducting

Polypyrroles for the Determination of Some Anthropogenic Organic

Pollutants in Waste Water, Abstract ID: Conf116a1624, ICMAT 2007

Conference, Singapore, 1-6th June, 2007.

243

7.4.2 Manuscripts and publications authored or co-authored from the PhD

study.

1. Akinyeye, R.0, Sekota, M., Baker, P. & Iwuoha, E. (2006). Chemical

Synthesis And Morphology of β-Naphthalene Sulphonic Acid Doped

Polypyrrole Micro/Nanotubes, Fullerenes, Nanotubes & Carbon

Nanostructures, 14, 49-55.

2. Akinyeye, R.O., Michira, M., Sekota, M., Ahmed, A-Al, Baker, P. &.

Iwuoha, E. (2006): Electrochemical Interrogation and Sensor Applications of

Nanostructured Polypyrroles, Electroanalysis, 18(24), 2441 – 2450.

3. Akinyeye, R.O., Michira, M., Sekota, M., Ahmed, A-Al, Tito, D. Baker,

P.G.L., Brett, C.M.A., Kalaji, M. & Iwuoha, E. (2007). Electrochemical

Synthesis and Characterization of 1, 2-Napthaquinone-4-Sulphonic Acid

Doped Polypyrrole, Electroanalysis, 19(2-3), 303–309.

4. Michira, M., Klink, M., Akinyeye, R.O., Somerset, V., Sekota, M., Ahmed,

A-Al, Baker, P.G.L. & Iwuoha, E.I. (2007). Anthracene Sulphonic Acid-

Doped Polyanilines: Electrodynamics and Application as Amperometric

Peroxide Biosensor, Chapter 5 in Recent Advances in Analytical

Electrochemistry, 0 – 00, ISBN: 978-81-7895-274-1, edited by Kenneth I.

Ozoemena (in press).

5. Akinyeye, R.O., Michira, M., Botha, S., Baker, P. & Iwuoha, E. (2007).

Electrocatalytic Sensor Applications of Nanostructured Polypyrroles and

Polythiophenes, Chapter 4 in Recent Advances in Analytical Electrochemistry,

0 – 00, ISBN: 978-81-7895-274-1, edited by Kenneth I. Ozoemena (in press).

6. Akinyeye, R., Anna Ignaszak, Al-Ahmed Amir, Priscilla Baker and

Emmanuel Iwuoha. (2007). Electrochemically Printed Polypyrrole

Impedimetric Nanosensor for Benzidine, EBASI 2007 Conference Paper

submitted for publication in Afican Physical Review Journal (in review).

244

7. Akinyeye, R.O., M., Klink, Ignaszak, A., Ahmed, A-Al, Baker, P., & Iwuoha,

E. (2007). Impedimetric Applications of Nanostructured Conducting

Polypyrroles for the Determination of some Anthropogenic Organic

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Nonostructured polypyrrole impedimetric sensors for ... · Michaelis-Menten constant ... found to exhibit good electronic and spectroscopic properties. ... carriers characteristics

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